Experiments were performed to determine whether both prolactin (PRL) surges are required to maintain pseudopregnancy (PSP) in the rat. Vaginal cycles were followed and animals that exhibited 2-3 normal cycles were cervically stimulated electromechanically to induce PSP. In one set of experiments the afternoon PRL surge was blocked by i.p. injection of 100 micrograms of 2Br-alpha-ergocryptine (CB-154) at 1200 and 1600h while the nocturnal surge was blocked by injecting 100 micrograms of CB-154 at 2400 and 0400h immediately after cervical stimulation. Pseudopregnancy was maintained only in those animals receiving CB-154 injections in the afternoon. In another series of experiments the initial CB-154 injection, given to block either surge, was delayed for one day. Once again only the nocturnal surge was capable of maintaining PSP in all animals. For those animals who had an initial nocturnal surge but all subsequent nocturnal surges were blocked, the afternoon surge was able to support PSP in only 50% of the animals. In those animals whose afternoon PRL surge was blocked, the duration of the nocturnal surge was extended so that it provided 85% of the total PRL produced by control PSP animals. In those animals whose nocturnal surge was blocked, the duration of the afternoon surge was also extended, however, in this case the extended surge only accounted for 58% of the total PRL produced by control PSP animals. The results indicated that when the diurnal PRL surge was blocked the nocturnal surge was extended so that the amount of PRL released was nearly equivalent to the amount released by both surges, however, when the nocturnal surge was blocked the extension of the diurnal surge did not provide adequate PRL to maintain PSP in all animals.