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) found that daily injections for 11 days of an antiserum prepared against purified sheep LH inhibited the oestrous cycle in mature female rats.The work reported here was designed to demonstrate whether antiserum to ovine LH administered at the time of the release of the ovulating gonadotrophic complex would prevent ovulation. LH was prepared from ovine pituitary glands by the method of Ward, McGregor & Griffin (1959), assayed by the ovarian ascorbic-acid depletion method of Parlow (1961) and shown to have the same order of potency as the NIH-S-LHX preparation.Antiserum to this LH preparation was prepared in two rabbits according to the method of Moudgal & Li (1961). A measure of the potency of the pooled antiserum was obtained by the graded neutralization of ovine LH in vitro by varying concen¬ trations of antiserum. The residual activity of the supernatant was estimated by the Parlow assay. Gel-diffusion studies following the method of Ouchterlony (1953) gave evidence for the presence of a single antigen-antibody system. Sexually mature virgin female rats of the Sprague-Dawley strain were used. Ovulation was confirmed by counting the ova in the fallopian tubes.The injections of antiserum were given at approximately the time of initiation of the release of the Ovulating gonadotrophic complex' from the pituitary, i.e. at approximately 12 hr. before the expected ovulation. A vaginal smear was obtained from each rat on the following morning before it was killed. The ovaries were fixed for subsequent histological examination. A preliminary experiment showed that the