It has been demonstrated in the rat that on the day of prooestrus there is a critical period for the initiation of the release of the 'ovulating gonadotrophic complex' from the pituitary gland (Everett, 1952). Thus, in animals kept under controlled lighting conditions with illumination from 5 a.m. to 7 p.m. the critical period is between 2 and 4 p.m., ovulation occurring between 1 and 2.30 a.m. the next day. Bourdel (1961) showed that rabbit anti-ovine-LH serum could neutralize endogenous LH in immature female rats, and more recently Bourdel & Li (1963) found that this antiserum, when injected into adult female rats daily for 12 days beginning 36 hr. in advance of presumed oestrus, could suppress this oestrus and subsequent ones. If the first injection were given only 12—16 hr. in advance, the first expected oestrus occurred normally, although subsequent oestrous periods were suppressed. Similarly, Young, Nasser & Hayashida (1963)