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and E. DOMASKI

The effects of lesions of the suprachiasmatic nuclei (SCN), anterior hypothalamic area (AHA) above the SCN and medial basal hypothalmus (MBH) on oestrous cycles in Merino ewes were observed. Ten animals with lesions of the SCN area continued to have oestrous cycles and ovulations throughout the whole breeding season; in six of these ewes the cycles and ovulations did not stop but were extended into the quiescent non-breeding period. Lesions placed above the SCN in the AHA (three animals) did not affect the oestrous cycles. Together with earlier experiments involving anterior hypothalamic deafferentation rostral and caudal to the SCN, it is suggested that the SCN has a role in transmitting inhibitory action from the medial preoptic area and suppressing the liberation of gonadotrophin-releasing hormone(s) (GnRH) during the non-breeding anoestrous period.

Injury to the MBH was followed by cessation of oestrous cycles and of ovulation, a decrease in the number and size of gonadotrophic cells, atrophy of the ovaries and a lowered basal concentration of LH in plasma. This was probably the result of injury to the neurones that produce or transport GnRH to the median eminence.