Further analysis has been made of the response of the rat pituitary gland to LHRH during the 4-day estrous cycle. LHRH was infused iv at a constant rate (50 ng/h) into phenobarbital-treated rats at different times during the estrous cycle. Infusion at this rate in proestrous rats simulates the rising and plateau phases of the spontaneous proestrous surges of LH and FSH in plasma. Plasma LH rose to similar heights during the "initial phase" of LH release (during the first 40 min of infusion) on the afternoons of estrus, diestrous day one, and proestrus and during the morning of proestrus. The increase during the afternoon of diestrous day two was significantly less than that in all the other groups. A similar response was seen in the case of FSH release. A "rapid rising" or "augmented" phase of LH release (during 40-120 min of infusion) was present in all groups and the magnitude of the response was greatest during the afternoon of proestrus. In the case of FSH, an augmented phase of release started 60 min after the start of infusion, and the response during the afternoon of proestrus was slightly greater than the responses measured at the other times tested. The responses on diestrous day one were not altered when phenobarbital was omitted or when rats were ovariectomized shortly before LHRH infusion. Other differences in the LH and FSH responses during both initial and augmented phases of release were seen in rats tested at different times during the estrous cycle with an LHRH infusion rate which caused a supraphysiological response on proestrus. The results suggest that 1) the initial rising phases in plasma LH and FSH during the spontaneous surges during proestrus are not the result of an increase in pituitary responsiveness to LHRH during the estrous cycle, 2) augmented phases of LH and FSH release can be elicited on all days of the estrous cycle, and 3) the increases in magnitude of the augmented phases of LH and FSH release on proestrus, as compared to those on other days of the cycle, are the result of an increase in pituitary responsiveness to LHRH during the estrous cycle.