Seventy patients treated with isoxsuprine for premature labor were studied. In patients with intact membranes, prolongation of pregnancy for more than 7 days occurred in 77% of women with 50% cervical effacement or less and 3 cm dilatation or less at the initiation of therapy, and in none with more than 50% effacement and more than 3 cm dilatation. Cervical effacement was the primary factor in determining success. Cord isoxsuprine concentrations averaged 90% of maternal concentrations at delivery. Maternal and cord isoxsuprine concentrations at delivery were inversely correlated with the drug-free interval before delivery. An interval of more than 5 hours was necessary to attain a cord concentration of less than 2 ng/ml, a level not associated with neonatal problems. Drug-free intervals of 2 hours or less usually resulted in cord isoxsuprine values of more than 10 ng/ml, levels that are associated with severe neonatal problems. Seventy-seven percent of infants with cord isoxsuprine concentrations of more than 2 ng/ml and 91% with values of more than 10 ng/ml were delivered of mothers with more than 3 cm dilatation or more than 50% effacement at the initiation or reinstitution of intravenous therapy. Most severe neonatal problems are preventable if patients are selected carefully.