The effects of treatment with estradiol-17 beta (E2) and progesterone (P4) on the neuronal system for gonadotropin-releasing hormone (GnRH) of the ovariectomized ewe during the non-breeding season were examined using immunocytochemical and radioimmunoassay analyses. The majority of GnRH-immunoreactive cell bodies are located in the medial preoptic area (POA). Clusters of labelled cell bodies also are found in the septum and anterior hypothalamus. GnRH fibers projected from the medial POA to a dense fiber plexus in the median eminence (ME). For ewes treated with E2, the percentage of GnRH cell bodies that were multipolar was approximately twice that for ewes not treated with E2 (P less than 0.05). The intensity of immunocytochemical staining in the GnRH cell body, and the incidence of fiber branching close to the cell body also were significantly greater for ewes treated with E2 (P less than 0.01 and P less than 0.05, respectively). The intensity for staining of GnRH in the ME appeared to be greater for ewes not treated with steroid, and this was reflected by a greater density of GnRH fibers in ewes not treated with steroid (P less than 0.05). The mass of GnRH in the ME of the ewes not treated with E2 was approximately twice that for E2-treated ewes; however, there was little difference in the total contents of GnRH in the hypothalamus and POA between E2 and non-E2-treated ewes.(ABSTRACT TRUNCATED AT 250 WORDS)