This article reviews the clinical and basic science investigations regarding the safety and efficacy of calcium channel blockers as tocolytic agents. The authors reviewed the English language literature on the pharmacology and clinical applications of calcium antagonists in obstetrics. A MEDLINE (1966-2000) search was performed with the terms "calcium channel blockers," "randomized controlled trial," "preterm labor," "calcium antagonist," "tocolysis," and "nifedipine." References from these data sources were then used to find additional studies. Animal data and clinical trials in humans were included. The safety of these agents was researched in published data from the nonobstetric as well as obstetric literature. The calcium channel blockers most commonly used as tocolytics are nifedipine and nicardipine. These agents act to inhibit calcium influx across cell membranes, thereby decreasing tone in the smooth muscle of the vasculature. They act as profound vasodilatory agents and have minimal effect on the cardiac conduction system. Numerous randomized clinical trials have shown them to be as effective as beta-mimetics and magnesium in achieving tocolysis. When used for tocolysis, calcium antagonists have fewer maternal side effects than other tocolytics and have no adverse effect on fetal outcome.