A specific and sensitive RIA was developed for measuring immunoreactive LHRH (IR-LHRH) in methanol acid-extracted human plasma. Heparinized blood samples were obtained from normal adult women and men, and the plasma was extracted with methanol, acidified, lyophilized, and stored at -20 C until assayed for LHRH. The lowest detectable levels of IR-LHRH in 1.0-3.5 ml methanol acid-extracted plasma ranged from 0.975-1.95 pg/assay tube, and recovery of synthetic LHRH from plasma averaged 88.6 +/- 2.4% (SE). Gel chromatographic separation analysis revealed comparable elution peaks for endogenous, tritiated, and synthetic LHRH. The plasma extract was also effective in evoking the release of LH and FSH in vitro using isolated rat hemipituitaries. IR-LHRH levels in human peripheral plasma rose 50-fold within seconds after an iv bolus injection of 100 micrograms synthetic LHRH with a return to baseline within 60 min. When the pattern of IR-LHRH secretion in peripheral plasma was studied, episodes of IR-LHRH rise were observed approximately every 60 min in both men and women. The physiochemical and biochemical properties of IR-LHRH suggest that it is an authentic LHRH. The presence of pulses of IR-LHRH suggests that its concentration in peripheral blood reflects changes in levels in the pituitary portal blood.