The effect of synthetic LHRH on serum levels of FSH, LH, and hCG was determined during early and midgestation. Eight healthy volunteers were studied during the first (8-9 weeks) and second (15-20 weeks) trimesters of pregnancy with 4 patients in each group. Serum samples, obtained before and 15, 30, 45, 90, 120, 150, and 180 minutes after an intravenous bolus of 100μg LHRH, were assayed for LH, FSH, and hCG by specific radioimmunoassays. Serum levels of estradiol, progesterone, and 17α hydroxyprogesterone were also measured in samples obtained prior to and 3 hours after LHRH injection. The serum levels of LH and FSH were undetectable in all samples, and no increase was observed after administration of LHRH. Levels of chorionic gondadotropin, estradiol, progesterone, and 17α hydroxyprogesterone did show slight fluctuations, but there was no significant effect from LHRH administration. These results support the concept that pituitary gonadotropic function is markedly suppressed during early human pregnancy.