Oxytocin causes the rapid tyrosine phosphorylation of mitogen-activated protein (MAP) kinase in both human and rat puerperal uterine myometrial cultured cells. The potential role of the MAP kinase pathway in oxytocin action was investigated with the specific MAP kinase kinase (MEK) inhibitor, PD98059. Oxytocin stimulation of the tyrosine phosphorylation of MAP kinase in both human and rat cultured puerperal uterine cells was abolished by pretreatment of the cells with MEK inhibitor in a dose-dependent manner. Although MEK inhibitor had no effect on oxytocin-induced intracellular Ca2+ mobilization in either pregnant human or pregnant rat uterine cells, it partly inhibited oxytocin-induced pregnant rat uterine contraction in a dose-dependent manner. These results suggest that MAP kinase pathway may have some important roles in oxytocin-induced uterine contraction.