Exposure to CS2, an organic solvent, is associated with an increased rate of abnormal labor or dysmenorrhea. Contraction of quiescent uteri during pregnancy can cause preterm labor. We wish to know the effects of in vivo and in vitro exposures to CS2 on uterine contractions of mid-gestation rats. After 10-d exposure to 300 or 600 mg/kg CS2, uteri of pregnant rats were measured for contractile responses to various stimuli, such as KCl, oxytocin, carbachol or A23187, a calcium ionophore, using standard muscle bath apparatus. CS2 treatment significantly increased the contractile response to KCl, carbachol, and A23187. The increase to A23187 was the greatest. In contrast, in vitro exposure to CS2 immediately suppressed carbachol-induced contraction but did not affect spontaneous and KCl-induced contractions. Results showed the pregnant uterus of the rat is susceptible to CS2. The influence of in vivo exposure to CS2 on uterine contraction was opposite to that in vitro. The increased response of CS2-treated uteri to A23187 suggests that in vivo exposure to CS2 may sensitize contraction machinery to calcium through indirect pathways.