Incubated pituitary halves from ovariectomized, estrogen-implanted female rats were shown to be much more sensitive to LHRH than pituitaries from castrated, nontreated animals. LHRH in a concentration of 1,885 pg/ml increased the release of LH and FSH from 7.3 ± 0.9 and 0.91 ± 0.13 ng/h/hemipituitary respectively to 21.4 ± 1.9 and 1.97 ± 0.18 ng/h in animals implanted with the steroid. In contrast, 5,000 pg/ml of LHRH increased LH secretion from 3.4 ± 0.3 to 8.4 ± 0.4 ng/h in ovariectomized, nontreated animals. In pituitaries from both steroid and nontreated animals a highly significant dose response for LH and FSH secretion to the actual concentration of LHRH measured in each incubation tube by radioimmunoassay was observed. When expressed as percent of the corresponding control release, maximal stimulation of LH and FSH was comparable. Pituitaries from implanted animals provided a very sensitive bioassay for LHRH, in which amounts of the peptide lower than 100 pg/ml were detected. The apparent responsiveness to LHRH of pituitaries from estradiol-treated rats was found to be over 20 times greater than that of pituitaries from nontreated cast