The effects of an intravenous bolus injection of D2343 (0.75 or 1 mg) or terbutaline (0.25 mg) on spontaneous term labor were studied. The uterine activity was recorded by a transducer-tipped intrauterine catheter and calculated electronically by an uterine activity integrator module built in a conventional fetal monitor. After drug administration the uterine activity was reduced to a similar extent with D2343 and terbutaline but the contractions were abolished for a longer time with terbutaline injection (mean of 9.3–9.8 min compared with 17.0 min). Maternal blood pressure was unaffected in both groups but the increase in pulse rate was slightly higher in the D2343 groups. D2343 seems to have an effect ratio between uterine inhibitory capacity and cardiovascular side effects similar to terbutaline.