In the study we evaluated the effects of infusion of exogenous leptin to the third ventricle of the brain on the expression of immunoreactive (ir) neuropeptide Y (NPY) neurons in the hypothalamus and ir gonadotrophin releasing hormone (GnRH) nerve terminals in the median eminence of prepubertal lambs in the conditions of short fasting. Merino female sheep (n=16) were randomly divided into four groups, two fed with standard feeds and two fasted for 72 h. One standard and one fasted groups were infused with Ringer saline (controls), remaining standard and fasted groups with leptin (25 microg/120 microl/h), for 4 h during three consecutive days, and then slaughtered. Ir NPY and ir GnRH were localized by immunohistochemistry using specific polyclonal antibodies. Detection of both hormones was followed by the image analysis and expressed as the percent area stained and integral density of immunostaining. In the hypothalami from all groups the ir NPY perikarya and varicose nerve fibers were localized in three distinct sub-areas, in the arcuate (ARC), paraventricular and periventricular nuclei. In fasted sheep the percent area and integral density for immunoreactivity of NPY increased significantly (P<0.001) in three sub-areas compared to the standard-fed animals. Leptin infusion lowered the both parameters (P<0.001) but solely in the ARC NPY population of fasted sheep. The percent area and integral density of immunostaining for ir GnRH in fasted sheep revealed the augmentation (P<0.001) compared to standard-fed sheep. Leptin infusions diminished (P<0.001) both parameters in fasted, without effects in standard-fed lambs. In conclusion, the enhanced by fasting immunoreactivity of the ARC NPY perikarya and varicose nerve fibers and restrained immunoreaction of GnRH terminals in the median eminence were reversed by exogenous leptin. It is suggested that leptin can affect GnRH release via ARC NPY neurons in conditions of deficit of nutrients in prepubertal, female lambs.