Activin is a pleiotropic growth factor with a broad pattern of tissue distribution that includes reproductive tissues. Although direct actions of activin have been described in gonadal and uterine tissues, actions in the myometrium have not been defined. In this study, we have characterized the responsiveness of uterine tissue and myometrial cell lines to activin-A. Uterine tissue and the myometrial cell line, PHM1 (pregnant human myometrial 1) and hTERT HM (hTERT-infected human myometrial) respond to activin-A as measured by phosphorylation of Smad-2. Those cell lines express a full complement of activin receptors, as well as activin _A subunit and follistatin. Activin inhibited proliferation of PHM1 and hTERT HM cells, with more extensive growth inhibition observed in PHM1 cells. In PHM1 cells, activin-A decreased oxytocin receptor and HoxA-10 mRNA expression, but did not alter total progesterone receptor, Cox-2, and connexin 43 mRNA expression levels. Furthermore, treatment of PHM1 myometrial cells with activin-A attenuated oxytocin and thromboxaneA2 induced intracellular Ca²⁺ accumulation. In conclusion, myometrial cells are activin sensitive and activin-A can regulate myometrial cell functions.