Department of Anatomy, Faculty of Science, Mahidol University, Rama VI Road, Bangkok, Thailand (Received 19 July 1978) Sex hormones have been shown to influence the levels of growth hormone (GH) in rats and mice (Birge, Peake, Mariz & Daughaday, 1967; Callahan, Somana & Srikhao, 1972; Sinha, Selby, Lewis & VanderLaan, 1972). Since treatment of neonatal female rats with androgens changes the pattern of gonadotrophin secretion from the cyclic female type to the non-cyclic male type (Gorski, 1971; Dorner, 1977), it was thought to be of interest to discover whether this treatment alters the pituitary GH content in female rats to levels similar to those found in male rats before and after puberty and whether after neonatal orchidectomy there is a reduction in the pituitary GH content to levels found in normal female rats. Newborn rats of a Fisher strain were divided into four groups: normal male, orchidectomized male and normal