The effects of the selective beta-2-receptor stimulator terbutaline on the activity of gravid, human myometrium were investigated in vitro and in vivo, before and after administration of different beta-receptor blockers. Terbutaline, 0.2-1.0 mu-g/ml, inhibited the spontaneous contractile activity of isolated strips of myometrium. This effect was unaffected by the selective beta-a-receptor blockers practolol, 1 mu-g/ml, and H 93/26, 1 mu-g/ml. However, the non-selective blocker propranolol, 0.1 mu-g/ml, completely inhibited the terbutaline effects. The in vitro effects of terbutaline could be correlated with findings in vivo. Intra-uterine pressure was recorded in 4 pregnant women at term. Infusion of terbutaline, 10-15 mu-g/min, for 20-40 min, effectively inhibited both spontaneous and oxytocin-stimulated uterine activity. There was a moderate increase in maternal heart rate, but no consistent effect on maternal blood pressure. Fetal heart rate was little affected. The uterine effects of terbuline were not influenced by practolol, 5-20 mg i.v., but completely inhibited by propranolol, 1-2 mg i.v. The results suggest that terbutaline inhibits uterine motility by effects on uterine beta-2-receptors and that it can be given in clinically effective doses without adverse circulatory effects on mother or fetus.