Eminent environmental temperature has adverse affect on dairy cattle production and reproduction. The periparturient period is also very critical for reproductive functions of dairy cows. A variety of methods are used to improve fertility in these stress conditions. The present study was conducted under a series of three experiments to find out stress-reproduction relationship and effect of vitamin E on this interaction. Study-I, was conducted to assess physiological and reproductive changes, in relation to heat stress in different dairy cattle breeds in various seasons. A total of thirty six lactating dairy cows, nine from each breed of local (Sahiwal and Achai), Cross-bred (Sahiwal x HF) and Holstein Friesian were selected. Sampling was conducted when the cows were in the di-estrus phase of the estrus cycle in February (thermoneutral), April (thermotransitional) and June (thermal stress) at an average environmental temperature of 18°C, 32°C and 42°C respectively. A significant decrease in Daily milk yield (DMY) was observed in all breeds with increase in environmental temperature (P < 0.01). DMY also varied significantly (P < 0.001) among breeds. The HF dairy cows showed the highest mean DMY followed by cross-bred, Sahiwal and Achai at all the three seasons. BCS was not affected by environmental temperature; however, BCS varied among breeds significantly (P < 0.01). A significant increase in all the physiological parameters (rectal temperature, respiratory rate and pulse rate) was observed in all dairy cows with increase in environmental temperature during thermo transitional and summer seasons in April and June respectively. Thermal stress in June increased concentrations of glucose (P= 0.014), cortisol (P < 0.001), heat shock protein-70 (HSP-70) (P < 0.001) and follicular numbers (P < 0.01), while decreased progesterone concentrations (P < 0.01) in all four breeds as compared to February. DMY and progesterone level are negatively correlated with environmental temperature. Glucose and cortisol levels were positively correlated with each other and negative with Progesterone. Positive correlation was found between glucose, cortisol and HSP-70 with rise in environmental temperature. However progesterone was negatively correlated with these parameters at high environmental temperature. It is concluded that the intensity of changes was higher in HF and cross-bred dairy cows than local Achai and Sahiwal, suggesting that extra managemental and therapeutical support is needed to improve fertility of these breeds in tropical regions. Study-II, was designed to detect the occurrence of oxidative stress in dairy cows by determining the plasma malondialdehyde (MDA) concentration during the periparturient period and its possible relationship with dairy cows fertility. Thirty-six periparturient lactating dairy cows, nine from each breed of Sahiwal, Achai, Cross-bred (Sahiwal x HF) and Holstein Frisian were selected. Sampling was conducted on post partum day (P.P.Day) 60, 82 and 105 of the lactation stage during the diestrus period of the estrus cycle. DMY varied significantly with P.P Days and among breed (P < 0.001). Highest DMY was shown by HF followed by cross-bred, Sahiwal and lowest by Achai. The interaction (PP.Days x Breed) effect was also highly significant (P < 0.01) for DMY. Serum glucose concentration significantly increased with increase in PP.Days (P < 0.01). Breed also affected glucose concentration (P < 0.001). A decrease in MDA concentrations were observed with increase in PP.Days (P < 0.001) from P.P day-60 to 82 and 105. Breed also affected the MDA concentrations significantly (P < 0.001), more prominent in HF and cross-bred. Both P.P. Days and breed significantly affected the serum cortisol concentration (P < 0.001). The cortisol concentration significantly decreased from P.P day-60 to 82 and 105. Progesterone level increased in all breeds (P < 0.05) with increase in P.P. Days (P < 0.001) from day-60 to 82, continued up to day-105. BCS was negatively correlated with serum cortisol, glucose and progesterone. Cortisol, DMY and MDA were positively correlated with each other and negative with progesterone and P.P. Days. The study revealed that HF and cross-bred dairy cows were more prone to oxidative stress than local breeds. Correlations analysis indicates that high milk production delayed ovarian activity effecting the reproduction of periparturient cows as assessed by progesterone concentrations. Therefore, it is suggested that higher genetic merit milk breeds should give extra managemental support for their better productivity and fertility. Study-III, was designed to find out the response of different breeds to vitamin E supplementation in combating stress and improving reproductive cyclicity. The experiment was conducted on thirty six lactating dairy cows, nine from each breed of Sahiwal, Achai, Cross-bred (Sahiwal x HF) and Holstein Frisian. Vitamin E was supplemented to all dairy cows in feed at a dose rate of 1000 IU/ cow/ day. Sampling was conducted on day-zero (control) before Vitamin E supplementation, day-20 and day-40 of Vitamin E supplementation during the diestrus phase of the estrus cycle. Daily milk yield and BCS varied significantly among breeds (P < 0.001) with no significant effect of vitamin E. Serum glucose increased significantly (P < 0.05) with vitamin E supplementation, but did not varied among breeds significantly. There was significant increase in SOD and GPx with vitamin E supplementation (P < 0.001). Breeds also varied in super oxide dismutase (SOD) and glutathione per oxidase (GPx) activities (P < 0.05, P < 0.001 respectively) with more increase in GPx in local dairy breeds. Stress markers, plasma MDA and HSP-70 and serum cortisol concentrations decreased significantly (P < 0.001) with vitamin E supplementation with significant variations among breeds (P < 0.001). Progesterone concentration increased significantly (P < 0.001) in all breeds with vitamin E. Breed effect on progesterone concentrations was also found significant (P < 0.05). Local breeds showed higher progesterone concentration while overall increase was found higher in Cross-bred dairy cows (39.95%). Number of dominant follicles decreased significantly with vitamin E supplementation (P < 0.01) in all the dairy breeds, highest in cross-bred dairy breeds. Breed also showed significant effect on number of follicles (P < 0.05). DMY showed positive correlation with MDA and negative with SOD, GPx and progesterone. The stress markers (cortisol, HSP-70 and MDA) were positively correlated with each other and glucose concentration while negative with antioxidant markers. Antioxidant markers were positively correlated with progesterone. It can be concluded that improved milk breeds showed more prominent response to vitamin E supplementation than local breeds suggesting regular feeding of antioxidant to these breeds for better performance.