Fifty Surti buffalo heifers between 17 and 42 mo of age (n = 24, < 24 mo; n = 26, > 24 mo) were used to study the changes in body weight, prolactin (Prl), progesterone (P₄) and luteinizing hormone (LH) as a result of supplemental lighting. Buffalo were randomly assigned to either natural day light + 4 hr supplemental light (S; n = 25) or natural day light (C; n = 25) groups on d 11 of the experiment. Data on individual body weights (BW) and ovarian structures were obtained weekly beginning from d 1 and d 14, respectively. Jugular vein blood samples were obtained on alternate days from d 1 to d 10, and twice weekly from d 11 to d 98 to determine serum Prl, P₄ and LH. Between d 99 and d 105 blood samples were obtained at -30, -15, 0, 15, 30, 45, 60, 90, 120, 150, 180 and 240 min post gonadotropin releasing hormone (GnRH) administration (30 µg) to determine LH response. Meteorological data were recorded throughout the study. Data on conception of heifers were obtained within 6 mo of the end of the study. Day-light ambient temperatures and relative humidity generally were > 27 C and < 70%, respectively. Heavy precipitation during the 11th and 12th wk interrupted the drought. S group animals had 5.8% heavier (p < 0.01) BW, 6.1 kg net BW gain (p < 0.01), higher mean Prl (42.6 ± 0.2 vs 40.1 ± 0.2 ng/ml; p < 0.01), a Prl increase (35.5 ± 2.0 ng/ml to 46.0 ± 1.8 ng/ml vs 41.7 ± 2.3 ng/ml to 42.8 ± 2.2 ng/ml; p < 0.01), higher P₄ (0.39 ± 0.02 vs 0.18 ± 0.02; p < 0.07), and higher mean LH (0.52 ± 0.01 vs 0.46 ± 0.01; p < 0.06) than C. Older heifers had 67% greater BW (p <. 0.01) during the treatment period, but a net BW loss (-7.8 kg vs 4.6 kg; p < 0.01), higher mean Prl (49.0 ± 0.2 vs 33. 7 ± 0.2; p < 0.01), P₄ (0.39 ± 0.02 vs 0.18 ± 0.02; p< 0.07) and LH (0.54 ± 0.01 vs OA4 ± 0.01; p < 0.01) compared to younger heifers. Older light supplemented heifers had higher mean P₄ (0.39 ± 0.02 vs 0.18 ± 0.02; p < 0.07) than the other groups. Peak response to GnRH was reached at 30 min post GnRH. Older control heifers had highest peak response (p < 0.01) to GnRH. These weight and hormonal changes suggest that 4 hr supplemental light can stimulate body weight gain, pituitary function and ovarian activity in peri-pubertal buffalo under the existing planes of nutrition. High rainfall and humidity adversely affected (p < 0.01) BW. The S heifers had a greater rate of weight loss at rainfall above 60 mm than did C heifers. Rainfall had a negative effect on Prl and LH. Further investigations should be conducted to evaluate the optimum age range and environmental characteristics which affect supplemental light administration. / Ph. D.