The reliability of intrauterine pressure measurements was studied by placing two catheters in the same uterus in women in labor. Nineteen parturients were studied, five with two Intran I catheters in different pockets of amniotic fluid, five with two Intran II catheters in different pockets, and nine with two Intran II catheters in the same pocket of amniotic fluid. Among 1429 contractions studied, the catheters showed differences of pressure of less than 5 mmHg in 86% of contractions and of less than 10 mmHg in 94%. When cumulative uterine activity was calculated for each labor, the difference in the total active pressures shown by the two catheters was less than 5% in 17 cases and 5% or more in two cases. These differences are unlikely to be of any importance in the management of labor.