The mammalian female genital tract is innervated with nerve fibers containing vasoactive intestinal polypeptide (VIP), substance P, enkephalins and somatostatin. The effect of these peptides has been studied by in vitro tension recordings on smooth muscle preparations from the uterine body, cervix and Fallopian tube of eighteen women. Substance P (10(5) - 10(6) mol/l) had a dose-dependent stimulatory effect. VIP displayed a dose-dependently inhibitory effect on the substance P evoked contractions. Somatostatin, met-enkephalin and leu-enkephalin had neither stimulatory nor inhibitory effect. The findings suggest that substance P and VIP may participate in a dual nervous control of genital smooth muscle contractions.