The tocolytic and cardiovascular effects of intravenous nicardipine HC1, a calcium entry blocker, were studied in 20 pregnant rabbits. Premature uterine activity was induced by extraovular infusion of prostaglandin F2-α and three different nicardipine HC1 dose schedules were tested. Significant dose-related inhibition of both electrical and mechanical uterine activity was observed. Treatment was also accompanied by a significant rise in pulse rate and a decrease in both diastolic and systolic blood pressure. It is concluded that nicardipine HC1 is a potent tocolytic agent that deserves further evaluation for possible clinical use.