A radioimmunoassay for luteinizing hormone-releasing hormone (LH-RH) is described which is capable of detecting LH-RH in plasma and brain tissues. Fluctuations in plasma and hypothalamic LH-RH as well as plasma LH were measured in immature female rats in which ovulation was induced by a single injection of pregnant mare serum (PMS). High plasma levels of LH-RH were found 47–49 h after the initial injection of PMS, coincident with high plasma LH concentrations. However plasma LH-RH levels were considerably lower at 55–57 h post-PMS, when plasma LH levels were elevated. Hypothalamic LH-RH content could not be correlated with either plasma LH-RH or LH lev