Abstract
Neonatal treatment with sex steroids causes irreversible changes in the hypothalamic regulation of pituitary gonadotrophic function (Barraclough, 1967; Saunders, 1968). In male rats, injections of oestrogen in early postnatal life bring about an inhibition of spermatogenesis and a marked atrophy of the reproductive accessory glands because of a permanent suppression of pituitary gonadotrophic activity (Takewaki & Takasugi, 1953; Kind, Folchi Pi & Lasso, 1963; Arai, 1964; Harris & Levine, 1965). The present communication describes a study of the response of the hypothalamic—pituitary system to neonatal castration, using changes in pituitary cytology as criteria. A comparison was made between male rats injected or not injected with oestrogen from birth. Newborn male Wistar rats were divided into six treatment-groups (see Table 1). They were killed at 100 days of age. The pituitary glands, fixed in Bouin's fluid, were serially sectioned at 10 μ and different sections from all levels of each gland

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