Our laboratory previously demonstrated that the human myometrium contains functional hCG/LH receptors. The present study investigated whether hCG can directly regulate oxytocin-stimulated human myometrial contractions. Uterine specimens were obtained from 30- to 40-yr-old women undergoing hysterectomy for leiomyomata, metrorrhagia, or prolapse. Myometrial strips from the lower uterine segment were primed for 24 h with 2.2 nmol/L estradiol. Then, the slices were incubated for 4 h at 37 C with or without 10 nmol/L hCG and stimulated with 1 mumol/L oxytocin, and the contractions were measured. The results showed that hCG inhibited the amplitude while paradoxically increasing the frequency of contractions. The effect of hCG was seen in proliferative, but not secretory, phase myometrial specimens. hCG had no effect on rat hepatic portal vein smooth muscle contractions, suggesting that the hCG action was tissue specific. Oxytocin treatment of human myometrial smooth muscle cells resulted in a dose-dependent increase in intracellular free Ca2+ levels. Pretreatment with hCG resulted in an attenuation of the oxytocin response, suggesting that the action of hCG was mediated by decreasing intracellular free Ca2+ levels. In summary, our results demonstrate that hCG can directly inhibit the amplitude of oxytocin-stimulated contractions of human myometria from the proliferative phase of the cycle. The hCG action is tissue specific and appears to be mediated by decreasing intracellular free Ca2+ levels in myometrial smooth muscle cells.