The action and clinical significance of the vaginocavernosus reflex was studied in 17 healthy women with a mean age of 36.6 years. The response of the bulbo- and ischiocavernosus muscles to rapid vaginal distension by balloon was recorded by a needle electrode inserted in each muscle and displayed on the screen of an EMG apparatus. The two cavernosus muscles contracted on vaginal distension; the amplitude of contraction increased with the increase of volume of vaginal inflation. The reflex contraction was not evoked after frequent successive or slow inflations or upon inflation of the anesthetized vagina. The latency of the reflex decreased with increased volume of vaginal inflation. The reflex cavernosus muscle contraction on vaginal distension during the sexual act seems to enhance both the clitoris and penile erection. This is believed to result from the compression exerted on both the deep dorsal vein of the penis and the erectile cavernous tissue. Also, cavernosus muscles contraction may help to milk the semen in the penile urethra into the vagina while the penis is being withdrawn from the vagina after ejaculation. Furthermore, vaginal distension by the fetal head during delivery induces cavernosus muscles contraction, which is suggested to support the head during its descent and prevent vaginal and perineal tears.