Immunoneutralization of endogenous neuropeptide Y (NPY) with specific antibodies was used to evaluate the possible physiological role of this neuropeptide in the regulation of gonadotropin and PRL secretion in the mouse. Because regulation of anterior pituitary function can be profoundly altered by chronic GH excess, we felt it was also of interest to have examined the effects of immunoneutralization of NPY in transgenic mice expressing bovine (b) GH gene with mouse metallothionein-I promotor. Intact or castrated transgenic and normal (nontransgenic) littermate adult male mice were injected with control rabbit serum or anti-NPY rabbit serum (anti-NPY). Blood samples were obtained 24 h later for hormone measurements. In intact transgenic mice expressing bGH gene, plasma LH levels were reduced and plasma PRL levels were increased, as compared to their normal littermates. Plasma levels of FSH were similar in normal and transgenic animals. Injection of anti-NPY increased plasma LH levels significantly in both intact normal and intact transgenic mice. However, plasma levels of FSH and PRL were increased significantly after anti-NPY treatment only in intact normal mice. In both normal and transgenic males, castration lead to the expected increase in plasma FSH and LH levels, whereas plasma levels of PRL remained unaltered. Administration of anti-NPY to castrated animals was followed by a significant increase in circulating LH levels in both transgenic and normal mice, whereas plasma levels of PRL were not changed. The results suggest that NPY plays a physiological role in the control of gonadotropin and PRL release in the adult male mouse, and that expression of the bGH gene in transgenic mice is associated with altered role of endogenous NPY in the control of pituitary function.