Effects of magnesium sulfate and ritodrine hydrochloride on cardiovascular physiologic characteristics were studied in 70 human subjects treated for preterm labor. Systemic and uterine hemodynamic effects were investigated in five pregnant rhesus monkeys. Systolic blood pressure was minimally affected by either agent. Diastolic pressure, while not affected by magnesium sulfate, decreased 26.3% during ritodrine therapy. Maternal and fetal heart rates were minimally affected by magnesium sulfate. Ritodrine increased maternal and fetal heart rates significantly. In the monkeys, magnesium sulfate increased uterine and placental blood flows (by the microsphere technique) but failed to alter cardiac output. Ritodrine produced an increase in cardiac output but decreased perfusion pressure. Placental blood flow decreased by an average of 27.6%. Ritodrine would therefore seem contraindicated with a compromised fetal environment. Magnesium sulfate, by not altering perfusion pressure, may have a beneficial effect on uterine hemodynamics. These specific and distinct differences in cardiovascular and hemodynamic effects should be considered when either magnesium sulfate or ritodrine is selected as a tocolytic agent.