Neuropeptide Y (NPY) acts at the hypothalamus to regulate the reproductive function by stimulating the release of GnRH from hypothalamus. In the present study a group of 5 female adult rhesus monkeys (Macaca mulatta), 5.5-9 years old, mean body weight of 10.31±0.90 kg and with menstrual cycle of 31 days was used. Changes in their body weight, behavior and sex skin color were observed throughout the cycle. Menstrual cycle of each monkey was monitored daily by recording the onset and duration of menstrual bleeding with vaginal swabs. Baseline profile of estradiol (E2), progesterone (P), prolactin (PRL) and growth hormone (GH) were measured by collecting blood sample (2 ml) on different days throughout the menstrual cycle of 31 days. Sequential blood samples (2 ml) were collected at an interval of 15 minutes for one hour before NPY administration for the hormonal baseline and for 2 hours and 15 minutes after NPY administration. In order to study the effect of NPY on plasma E2, P, PRL and GH levels on day 1 (menstrual phase), day 7 (follicular phase), day 15 (peri-ovulatory phase) and day 21 (luteal phase) of menstrual cycle, 200 μg of NPY in single bolus intravenous injection was given. Individual and mean body weight during the menstrual cycle was not significantly different. After NPY administration monkeys were relaxed and comfortable. Sex skin coloration changed progressively from whitish pink to deep red following menstrual to periovulatory phase and then decrease in colour intensity occurred during luteal phase. Baseline profile of estradiol showed that plasma E2 concentration was significantly high (P<0.001) in the periovulatory phase of menstrual cycle compared to menstrual, follicular and luteal phases. The luteal phase plasma E2 level was significantly low compared to follicular phase (P<0.003) but not significantly different from the menstrual phase. Plasma estradiol level 15 minutes after NPY administration increased non-significantly in all the four phases of menstrual cycle compared to baseline at 0 minute. Then, subsequent significant temporal increase till 45 minutes on day 1, 75 minutes on day 15, 60 minutes on day 7 and day 21 followed by subsequent significant temporal decrease. At the end of experiment plasma estradiol attained the basal level in all the four phases. Baseline profile of plasma progesterone showed significantly low levels during menstrual, follicular and periovulatory phases compared to the luteal phase. No significant difference was observed in the plasma P concentration between menstrual, follicular, and ovulatory phases. In all the four phases of menstrual cycle plasma progesterone level 15 minutes after NPY administration increased non-significantly followed by significant temporal increase till 60 minutes on day 1, 105 minutes on day 7, 135 minutes (i.e. till the end of experiment) on day 15 and 30 minutes on day 21. After then non-significant temporal decrease on day 7 and significant on day 1 (P<0.0002) and day 21 (P<0.0007) was observed. The baseline profile of plasma PRL showed that plasma PRL levels were significantly high during menstrual (P<0.013) and periovulatory phases (P<0.023) compared to luteal phase. Plasma prolactin level of follicular phase was non-significantly lower than menstrual and peri-ovulatory phases. The plasma prolactin levels of follicular and luteal phases were not different. In plasma prolactin concentration after 15 minutes of NPY bolus injection a non-significant rise was observed on day 1 followed by non-significant temporal increase till 30 minutes and then significant temporal decrease till the end of experiment. On day 7 non-significant and on day 15 significant increase in plasma prolactin level was observed 15 minutes after NPY administration followed by significant temporal decrease on day 7 (P<0.0005) and day 15 (P<0.009). On day 21 a non-significant decrease in plasma prolactin level after 15 minutes of NPY administration followed by significant temporal decreased till the end of experiment. Regression analysis of variance showed highly significant temporal decrease (P<0.0003). The base line plasma in all the four phases of menstrual cycle GH levels in all the four phases of menstrual cycle were non-significantly different (P>0.05). NPY administration inhibited the plasma GH level in all the four phases of menstrual cycle. On day 1 (menstrual phase) of menstrual cycle plasma growth hormone level 15 minutes after NPY administration decreased non-significantly with subsequent non-significant temporal decrease till 45 minutes followed by significant temporal increase till the end of experiment. A highly significant decrease in plasma GH level was observed on day 7 (follicular phase) and non-significantly on day 15 (periovulatory phase) and day 21 (luteal phase) of menstrual cycle 15 minutes after NPY administration followed by non- significant temporal decrease on day 7 and day 15, but significant temporal decrease on day 21 (P<0.004) till the end of experiment. These results show that NPY has stimulatory and inhibitory effects on the ovarian and pituitary hormones by acting as a modulator, neurotransmitter and neurohormone. NPY has applications in pharmacological fields and can be used for further research.