Previous studies have shown that pentobarbital administration at 2 P.M. of proestrus i n the rat blocks ovulation and the drop in pituitary LH content normally seen at estrus; these data were interpreted as indicating that the drug acted by blocking LH release. Signs of prolongation of estrogen secretion were seen in these rats and, in the absence of further treatment, ovulation occurred 24 hours later. In the present study a single dose of anti-ovine-LH-serum administered at 1 P.M. of proestrus also blocked ovulation, but did not prevent the drop in pituitary content on the morning of "estrus." Therefore, one may conclude that this blockade was the result of preventing the peripheral effects of LH on the ovary.The antiserum blockade did not produce prolongation of estrogen secretion and no rat ovulated within the next four days. During this time pituitary LH content was normal, as were ovarian and uterine weight, although large follicles, rather than recent corpora lutea, were seen in the ovary. Six rats were followed until the next vaginal cornification (by 6 days); only three of the six had ovulated by day seven.The contrast between the sequelae of pentobarbital and antiserum blockade is interpreted in the light of a new theory of regulation of reproductive cyclicity in the rat.In the rat exposed to an alternating light-dark environment (lights on 5 A.M. to 7 P.M.) the release of the ovulatory surge of LH starts after 2 P.M. on the afternoon of proestrus, as shown by timed hypophysectomy studies (Everett, '61 ; Hoffmann and Schwartz, '65a). Much evidence suggests that this release occurs as the result of two "necessary and sufficient" conditions (Schwartz and Bartosik, '62): first, there appears to be a daily facilitatory period for LH release starting after P.M. (Everett, '61); second, enhanced ovarian secretion occurs on the day before proestrus (Schwartz, '64; Schwartz and Talley, '65). However, these two conditions are not completely independent since the critical period for LH release is prolonged when steroid secretion is enhanced (Hoffmann and Schwartz, '65a,b). The LH release for ovulation can be blocked by administration of pentobarbital before 2 P.M. of the day of proestrus, as shown by the effect of pentobarbital in preventing the proestrous rise in plasma LH (Schwartz and Caldarelli, '65), the drop in pituitary LI-I content normally seen at estrus (Schwartz, '64) and, of course, the expected ovula- The present study approached the problem of ovarian-pituitary interrelationships from a different point of departure. It has been demonstrated that a single dose of anti-ovine-LH-serum, when administered before the proestrous facilitatory period, can block ovulation (Kelly, Robertson and Stansfield, '63). The purpose of the present experiment was, first, to ascertain whether the antiserum blockade was peripheral (by testing to see if pituitary LH content depleted at estrus) and, second, to follow the subsequent ovarian and pituitary events and contrast these with the effects of pentobarbital blo...
