The main aim of this study was to examine the role of the hypothalamus in controlling the secretion of GH in the ewe. This was evaluated by studying the effect of lesions placed either in the anterior or the posterior medial-basal hypothalamus (MBH) on the concentration of GH in the peripheral circulation during pregnancy and lactation, i.e. when the levels would be high in normal ewes. Simultaneously, the level of prolactin in the peripheral blood of these animals were followed. Lesions of the MBH resulted in a marked decreasing in circulating GH as well as disturbances in the mammogenic and lactogenic processes during the periods of periparturition and lactation respectively. The changes were particularly evident if the anterior MBH was lesioned. The present experiments confirm our previous findings that a stimulatory centre is localized in the anterior MBH of sheep whilst in the caudal MBH there is an inhibitory centre regulating the release of prolactin. The results also confirm the important role of GH during lactation in ewes, especially during lactogenesis. The results indicate a differentiated but synchronizing and synergistic role of the MBH regulating the patterns of GH and prolactin secretion in late pregnancy and lactation.