Recently, vasoactive intestinal polypeptide (VIP) has been localized in nerve fibers in the human female genital tract. In the present investigation, the effect and concentration of VIP was studied in uterine tissue from pregnant and nonpregnant women, and the plasma concentration of VIP was measured in relationship to diurnal rhythm, intake of food, menstrual cycle, pregnancy, labor, age, and sexual arousal. In vitro VIP inhibited the contractions of the nonpregnant but not of the pregnant uterus. The median concentration of VIP in myometrium from pregnant women (less than 0.1 pmole/gm) was significantly lower than that in myometrium from nonpregnant women (1.6 pmoles/gm). The venous plasma concentrations of VIP during labor (10.5 to 13.0 pmoles/L) were significantly higher than those during pregnancy (2.0 to 5.0 pmoles/L) and the menstrual cycle of VIP increased significantly during sexual arousal, from 4.0 to 8.5 pmoles/L. The median arterial and venous concentrations in the umbilical cord (12.5 and 14.5 pmoles/L, respectively) were significantly higher than the concentration in maternal peripheral venous blood (5.2 pmoles/L). The plasma concentrations of VIP were not related to intake of food, diurnal rhythm, menstrual cycle, or age. The conclusion is that the function of VIP may be related to pregnancy, delivery, and sexual stimulation.