A single injection of beta-endorphin antiserum into mature male rats produced approximately a 3-fold rise in plasma LH levels by 10 to 50 min, and declined by 90 min, but remained significantly elevated above pre-injection values. Control rats injected with normal rabbit serum showed no elevation in plasma LH values. These results indicate that beta-endorphin is one of the endogenous opiates that tonically depresses basal release of LH in mature male rats.