Strips of myometrium were obtained at operation from eight nonpregnant and ten pregnant patients. The tissues were immediately suspended in a muscle bath and induced to contract isometrically with K2SO4in the presence and absence of halothane (0.5 to 2.0 vol per cent). The anesthetic produced significant dose-related depression of developed tension and rates of increase and decrease of tension in both pregnant and nonpregnant muscles. Time to peak tension was increased in the nonpregnant but not in the pregnant muscles. The depressant effect of the lower halothane concentrations (<1.1 vol per cent) was significantly greater in the pregnant muscles.