The possibility that beta-endorphin, an endogenous opiate, is involved in the regulation of the menstrual cycle was examined. Daily serum beta-endorphin levels, in conjunction with luteinizing hormone, progesterone, and 17 beta-estradiol were measured during 26 hormonally normal menstrual cycles. Twenty-one cycles showed a preovulatory peak and postovulatory trough of beta-endorphin, 2 cycles had a postovulatory peak, and 3 had a postovulatory peak with sustained elevation. The raw data were standardized by conversion to "Z-scores," and the composite values were computed for each of the three classes described above. Significance within these three classes was assessed using a one-way analysis of variance with an F-ratio at 95% confidence limits. The composite plot of the 26 cycles showed a statistically significant preovulatory peak occurring 2 days prior to the luteinizing hormone surge and a postovulatory trough of beta-endorphin 5 days later. These results suggest that beta-endorphins play a significant role in the neurochemical mechanisms of gonadotropin release.