Currently available tocolytic agents have shown partial or transient efficacy on uterine contractions delaying delivery for 24 or 48 h without reduction in perinatal morbidity and mortality. These facts led us to study a new tocolytic compound. A double-blind dose finding study of the nitroglycerin tocolytic effect during preterm labor was conducted using a continual reassessment method among six different doses (0.2-1.2 mg/h for 2 h) in pregnant women who were unresponsive, intolerant to salbutamol, or with a contraindication to this therapy. Twenty-five pregnant women were included. The probability of success in stopping uterine contractions reached, for the maximal dose (1.2 mg/h), only a 54% rate (95% CI: 29-79%). Twelve patients complained of headaches and 16 experienced a decrease in arterial blood pressure suggesting that it would not be safe to increase the dose in order to obtain a higher success rate. The present study suggests that nitroglycerin is not as effective as expected in controlling uterine contractions during severe preterm labor in patients where beta2 agonists cannot be used.