Sodium pentobarbital (50 mg/kg) was administered oneach day of the estrous cycle to 4- and 5-day cyclic Long-Evans rats. In rats with 4-day cycles, the incidence of ovulation was significantly decreased when barbiturate was administered as a single injection at various times from 12.45 h on estrus to 12.45 h on diestrus-2. A delay in vaginal cornification and uterine ballooning measured on estrus, as well as decreased sexual receptivity on proestrus, was also found to result from barbiturate injections on diestrus-1, indicating a suppression of estrogen secretion. The deleterious effects of barbiturate treatment could be reversed by the simultaneous administration of gonadotropin. A generally similar pattern of blockade by barbiturate during this time period was found in 5-day cyclic rats, although the inhibitory effects were less than in 4-day cyclic rats. Furthermore, no inhibition of ovulation was found following drug administration on diestrus-3. The pattern of ovulatory blockade by barbiturate on the afternoon of proestrus was also generally similar in the 4- and 5-day cyclic rats. Again, however, more complete suppression was found in the 4-day cycle, and the duration of the inhibitory effects was more prolonged. It was concluded that the inhibitory effects of barbiturate administered early in the estrous cycle (estrus, diestrus-1 and diestrus-2) probably reflect a daily pattern of neural activity which is causally related to gonadotropin and estrogen secretion necessary for the final occurrence of ovulation.

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