Sexual receptivity was suppressed in female rats immediately after the male had ejaculated. The inhibition was prevented by intravaginal injection of naloxone (100 micrograms) before testing and intravaginal injection of beta-endorphin (1.0 micrograms) inhibited sexual behavior in female rats in a manner comparable to that of ejaculation by the male. beta-Endorphin was present in ejaculatory plugs collected from the vagina of female rats (8.2 +/- 0.6 pM) and in seminal fluid collected from male rats (7.7 +/- 0.2 pM). The results suggest that beta-endorphin in the ejaculate of the male rat can act on the reproductive tract of the female rat to suppress her sexual behavior.