Neuropeptide Y (NPY) has recently been localized in the rat hypothalamus. In order to evaluate the putative effects of NPY on pituitary function, its action was examined on anterior pituitary cells in culture. Also, an immunocytochemical method was used with the aim of localizing endogenous NPY-like material at the cellular and subcellular levels of the pituitary gland. In vitro studies using dispersed anterior pituitary cells indicated that NPY (10–6 to 10–9M) increased the secretion of luteinizing hormone, growth hormone and prolactin, whereas β-lipotropin hormone and thyrotropin secretions were not affected. The presence of endogenous NPY was demonstrated in gonadotrophs, somatotrophs, corticotrophs and some lactotrophs, but not in thyrotrophs. In immunoreactive cells, NPY-like material was detected in the cytoplasmic matrix, in the secretory granules and in the nucleus distributed primarily in the euchromatin, in the vicinity of the heterochromatin. NPY-like immunoreactivity was also observed at the plasma membrane but only scarcely. These biochemical and immunocytochemical results indicate that NPY may play a direct regulatory role in adenohypophyseal secre