Neuropeptide Y (NPY) provides an important hypothalamic link between nutritional status and neuroendocrine mechanisms regulating growth and reproduction. The objective of the following series of experiments was to determine the effects of single or continuous administration of NPY on secretion of luteinizing hormone (LH) and (or) growth hormone (GH). In experiment 1, four ovariectomized (OVX) ewes and four OVX+estrogen-treated ewes each received, in a 4x4 Latin Square arrangement of treatments, a single injection of 0, 0.5, 5, or 50μg NPY via an intracerebroventricular (i.c.v.) cannulae to determine the effects on secretion of GH. NPY significantly elevated serum GH at the 50μg dose regardless of estrogen exposure (P=0.003). In experiment 2, eight OVX ewes were infused i.c.v. with NPY or saline (n=4/trmt) continuously for 20h in a linearly increasing dose, ending at 50μg/h NPY. Blood samples were collected via jugular cannulae every 10min during hour -4-0 (interval 1, pre-treatment), hour 6-10 (interval 2) and hour 16-20 (interval 3) relative to the initiation of infusion (0h). Mean LH and LH pulse frequency were lower in NPY- versus saline-infused ewes during intervals 2 and 3 (P<0.01), but NPY had no discernable effect on serum GH (P>0.10). In experiment 3, four OVX ewes were continuously infused with NPY as in experiment 2, except that the maximum 50μg/h dose was achieved after only 10h of infusion. Blood samples were collected every 10min, beginning 4h before and continuing until 4h after the NPY infusion. Mean serum LH changed significantly over time (P=0.0001), decreasing below pre-treatment levels by hour 3 of NPY infusion (P<0.01), and returning to pre-treatment concentrations following the end of infusion (P>0.15). Serum GH also changed significantly over time (P<0.001). Mean GH levels tended to be greater than pre-treatment levels by hour 2 of infusion (P<0.08), but thereafter returned to basal levels. Serum GH also increased following the end of NPY infusion (P<0.03). From these data we conclude that NPY exerts a persistent inhibitory effect on secretion of LH, and may stimulate the secretion of GH during the initiation and cessation of infusion of NPY. These observations support a role for NPY in mediating the effects of undernutrition on both LH and GH, and also provide evidence for potential mechanisms by which leptin, acting through NPY, may stimulate the secretion of GH.