We have investigated the effect of surgical isolation of the mediobasal hypothalamus of ovariectomized ewes on LH secretion (basal level, rhythmicity and oestradiolinduced surge) and progesterone-oestradiol induced sexual receptivity. The disruption of the posterior inputs (just anterior to the mamillary bodies) was without any endocrine or behavioural consequence. The bilateral disruption of antero-dorsal inputs resulted in the suppression of OB-induced LH surge and damping of the mean basal

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... vel. This effect was observed both shortly after surgery, and 3 months later (2 artificial cycles in each case). The pulsatility of LH basal secretion remained evident regardless the drastic damping of the level. Such results suggest that antero-dorsal inputs, coming from preoptic-suprachiasmatic areas are required both for the positive feed-back of oestrogens on LH secretion, and for the maintenance of the basal level of secretion. The persistence of the pulsatile pattern of secretion suggests that the mechanism involved could be located in the medio-basal area. In 8 cases the normal sexual receptivity induced by progesterone-oestrogen treatment was maintained despite elimination of the LH surge. In the 5 others, both sexual behaviour and LH surge were suppressed. In such cases the sections extended more dorsally, suggesting the importance of the dorsal part of the anterior mediobasal hypothalamus for the organisation of sexual behaviour. As first described by Haldsz and Pupp (1965), deafferentation of the hypothalamus has proved to be a highly valuable method of investigating the role of the brain in the regulation of endocrine functions of numerous species. In the rat, surgical disconnection of hypothalamic areas demonstrated that the mediobasal hypothalamus (MBH) regulates tonic secretions of gonadotrophins whereas the anterior hypothalamus-preoptic area is responsible for cyclic LH release which triggers ovulation. In the monkey, the work of Krey et al. (1975) tends to suggest that the two main functions regulating the secretion of gonadotrophins, i. e. tonic release and preovulatory surge, may be localized in the MBH. However Norman et al. (1976) using the same techniques, suggested a possible role for a more anterior part of the brain. In the sheep, the progesterone-oestrogen treatment which induces sexual receptivity in the ovariectomized female (Robinson, 1954) has been reported to result in a