We tested the hypothesis that leptin could prevent fasting-mediated reductions in pulsatile secretion and modify GnRH-mediated release of LH in heifers approaching puberty. Thirteen crossbred, prepubertal heifers (13.5–16 mo; 280–350 kg) exhibiting frequencies of pulses of LH between 0.67 and 1 pulse/h, were assigned randomly to two groups: 1) control (n = 6), fasted for 72 h with s.c. injections of saline at 12-h intervals, and 2) leptin (n = 7), fasted for 72 h with s.c. injections of oleptin (19.2 μg/kg) at 12-h intervals. Blood samples were collected intensively for 6 h on Days 0 and 3. This was followed on Day 3 with sequential administration of physiological (0.0011 μg/kg, i.v.) and pharmacological (0.22 μg/kg, i.v.) doses of GnRH and additional blood sampling. Leptin treatment increased (P = 0.0003) plasma concentrations of leptin 5–6-fold compared to controls. Fasting caused a marked decline (P = 0.01) between Days 0 and 3 in the frequency of LH pulses in controls; however, this effect was prevented in the leptin group, with pulse frequency increasing (P < 0.008) from Day 0 to 3. Leptin treatment increased GnRH-induced release of LH at both low (P = 0.04) and high (P = 0.02) doses. Plasma insulin and insulin-like growth factor-1 were reduced by fasting and unaffected by leptin. Leptin increased mean concentrations of growth hormone. Results indicate, for the first time, that exogenous leptin can prevent fasting-mediated reductions in the frequency of LH pulses and modify GnRH-mediated release of LH in intact, prepubertal heifers.