Low levels of endogenous LHRH-like immunoreactivity (IR) were consistently detected in methanolic extracts of plasma samples from anestrous and ovariectomized sheep. Mean concentrations were similar in magnitude in the intact and castrate ewes (7.7 +/- 7.0 and 6.1 +/- 0.8 pg/ml, respectively) but revealed greater within animal fluctuations in the ovariectomized sheep (CVs: anestrous ewes = 20 +/- 3%; ovariectomized ewes = 45 +/- 10%). Patterns of plasma LHRH-like IR and LH levels were not obviously related. Differences in levels of LHRH-like IR did exist between ewes and in the ovariectomized sheep the levels were positively correlated (r = .83, P less than ,05) with the pituitary LH response to synthetic LHRH injection (0.25 micrograms). Large plasma volumes (250 ml) were extracted using chloroform, methanol and acetic acid and then chromatographed using Sephadex LH-20. Plasma LHRH-like IR could be concentrated and was found to have a column elution volume coincidence with that of synthetic LHRH. These data indicate that there is a substance in ovine plasma which is close in size, solubility and IR to sLHRH. It does not reflect LH levels in either magnitude or pattern but does exhibit a coincident increase in variation with plasma LH levels following ovariectomy and appears to be associated with the pituitary responsiveness.