Fifty inbreds of maize from diverse origin were screened in the growth-chamber and glass-house for seedling parameters under both normal and high temperature regimes. Six parental lines were earmarked on the basis of various agro- physiological seedling traits under high temperature stress. The parental lines were sown in the field using diallel mating design to make all possible cross combinations. The parents, F1’s and their reciprocals were sown in the field in two sets using a triplicate RCBD. Normal irrigations were applied to both sets of experiment. Normal experiment was sown on Feb. 8th, whereas stress experiment was sown on March 8th. Data pertaining to various morpho-physiological parameters were recorded at different crop growth stages and subjected to statistical analysis. Significant mean square values for all the parameters under normal and heat stress environments revealed considerable genetic variability. Scaling tests were performed to check adequacy of data for analyzing additive-dominance model. Under both temperature regeims, all traits except RCI (fully inadequate) were fully adequate to additive-dominance model. Inheritance of all agro-physiological parameters was influenced by overdominant type of genetic effects as male × female interaction was also found significant for all parameters under both temperature regimes. Significant mean square values were observed for GCA & SCA effects under optimal and stress conditions in maize. Existence of highly significant GCA and SCA effects revealed that trait inheritance was controlled by additive, dominance and over-dominance type of genetic attributes under optimal and high temperature regimes. Reciprocal effects were highly significant for DTS, OP, PH, DTM, GDDs to 50 % silking and GDDs to 50 % maturity under optimal environmental conditions, while under stress environment significant reciprocal attributes were expressed by DTS, ELA, OP, PH, GDDs to 50 % silking and GDDs to 50 % maturity, depicting that inheritance of these traits is influenced by maternal or cytoplasmic contents. Magnitude of SCA variance was higher than GCA variance for all studied traits under both temperature regimes, due to which GCA/SCA variance ratio was less than unity indicating that all parameters were controlled by non-additive (dominance & over-dominance) type of genetic effects except leaf osmotic potential, whose inheritance was influenced by partial dominance type of genetic attributes under high temperature. Inbred lines D-103, NCIL-20-4 and NCIL-10-5 were best suited early maturing inbreds under both environments. Inbred lines NCIL-10-5, D-135 and NCIL-20-4 earmarked as high yielding parents under both temperature regimes. Three F1 crosses D-103 × NCIL-30-5, NCIL-20-4 × NCIL-10-5 and NCIL-10-5 × NCIL-30-5 were tagged as best combiners for most of the physiological, grain yield and yield related parameters under both environmental conditions. Heterotic estimates were studied for all traits under normal and high temperature. Under normal and heat stress environment, cross combinations D-103 × NCIL-30-5, NCIL-20-4 × NCIL-10-5 (its reciprocal) and NCIL-20-4 × WA3748 expressed maximal HP heterosis estimates for grain yield. Pioneer hybrid P-1543 used as commercial check to estimate standard heterosis for GPE, 100-KW and grain yield per plant under both environmental conditions. Results revealed cross combinations NCIL-20-4 × NCIL-10-5, its reciprocal and NCIL-10-5 × NCIL-30-5 as best F1’s due to their high performance over commercial hybrid. Under both temperature regimes, all seedling parameters were significantly interrelated with each other except root-shoot ratio. Under optimal and heat stress growth conditions in the field, agronomic parameters showed significant correlation with grain yield except days to 50 % tasseling, days to 50 % silking, ASI and days to 50 % maturity. All physiological parameters showed significant interrelation with grain yield except GDDs to tasseling, silking, GDDs between anthesis-silking and maturity.