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Department of Anatomy and Brain Research Institute, UCLA School of Medicine, Los Angeles, California 90024, U.S.A.

(Received 21 July 1977)

The existence of a 'critical period' in the ovulatory surge of pituitary gonadotrophin during the afternoon of pro-oestrus was demonstrated by Everett, Sawyer & Markee (1949). It was assumed that during this period, between 14.00 and 16.00 h, an amount of gonadotrophin sufficient to cause ovulation was released from the anterior pituitary gland. Administration of a number of neuropharmacologically active drugs before or during the 'critical period' was shown to inhibit the ovulatory response (Everett, 1961). Recently, Blake (1974) described the existence of an 'activation period' as well as a 'potential activation period' for the preovulatory release of luteinizing hormone (LH) in female rats. The 'activation period' is an extended 'critical period', from 14.00 to 17.00 h, during which time a neurohumoral signal continues to stimulate the anterior pituitary gland