The pulse of prolactin, present in female tammars kept in an inhibitory photoperiod and absent 5 days after a change to a stimulatory photoperiod, was artificially maintained during stimulatory photoperiods (Exps 1A and 1B) or abolished during an inhibitory photoperiod (Exp. 2). In Exp. 1A, thyrotrophin-releasing hormone (TRH) was injected i.v. for 8 consecutive mornings and in Exp. 1B ovine prolactin was injected i.v. for 10 consecutive mornings beginning on the day the photoperiod was changed from 15L:9D to 12L:12D. In Exp. 1A, in 5 of 6 control females the prolactin pulse was present until the 3rd morning after the lighting was changed, and absent thereafter. In both experiments the early progesterone peak and the interval to birth in the treated females was significantly delayed compared to the control females; reactivation of the CL did not occur until treatment ceased. In Exp. 2, for 5 consecutive mornings, before the prolactin pulse was expected to occur, bromocriptine was injected i.m. In 2/4 treated animals the prolactin pulse was abolished and these animals showed an early progesterone peak and gave birth 28 and 29 days after treatment began. The remaining 2 animals and the controls (N = 4) did not show reactivation until after they experienced a stimulatory photoperiod. The results indicate that the prolactin pulse is the critical factor in the maintenance of the inhibition of the CL of the tammar during seasonal quiescence.