beta-Sympathomimetic drugs cross the placenta freely. Just as these agents cause serious cardiovascular changes in the mother, they may cause severe cardiovascular complications in the fetus. beta-Sympathomimetic agents for tocolysis have been associated with fetal heart rate and rhythm disturbances, hydrops, stillbirth, neonatal cardiac failure, myocardial ischemia and infarction, and neonatal death. Prospective studies have documented changes in interventricular septa of babies exposed to these drugs. Histologic changes have been reproduced in animal models and in vitro similar to those seen in infants with myocardial disease caused by beta-mimetic therapy. The mechanism of beta-mimetic toxicity appears to be increased myocardial intracellular calcium leading to overexcitation and cell necrosis. Since serious fetal cardiovascular effects may occur with beta-mimetic use, benefits should clearly outweigh risks before these drugs are administered.