The ability of the pituitary to release LH in response to synthetic LHRH was tested in lactating female rats on days 7 and 17 post partum, and compared with that of normal cycling female rats on diestrous day 2 (controls). Three consecutive injections of LHRH (100 ng/100 g BW, sc), each 50 min apart, were given to each rat and sequential blood samples were collected at 25-min intervals by cardiac puncture under light ether anesthesia. In all 3 groups, the 2nd and 3rd injections of LHRH produced much greater increases in serum concentrations of LH than the 1st injection. However, this self-priming effect of LHRH on LH response was markedly attenuated in the postpartum lactating rats (PPL), compared with the normal cycling female rats on diestrous day 2. Three consecutive injections of LHRH produced significantly less LH release in PPL rats than in normal cycling female rats on diestrous day 2. Both day 7 and day 17 PPL rats released equally small amounts of LH in response to LHRH administration. The total amount of LH released by anterior pituitaries (APs) during a 5 h incubation in medium-199, from day 7 or day 17 in PPL rats, was significantly less than that released by the APs from normal cycling female rats on diestrous day 2. APs from day 7 and day 17 PPL rats also released less LH in vitro in response to LHRH (50 ng) stimulation than APs from normal cycling female rats. When APs from normal cycling female rats on diestrous day 2 were incubated with LHRH, the increments in LH release were greater at the end of the 2nd and 3rd h than after the 1st h of incubation. However, such increments in LH release were relatively small when APs from day 7 or day 17 PPL rats were similarly incubated with LHRH.