Effects of selective beta-adrenoceptor agonists on rectosigmoid motility during prolonged rectal distension were studied in 12 healthy volunteers in a double-blind, randomized fashion. Continuous distension was performed with a balloon in the proximal part of the rectum. Pressure was recorded by this balloon and by a catheter in the sigmoid. Contractile activity was quantified for three consecutive periods of 25 min. On separate days prenalterol (beta-1 agonist), terbutaline (beta-2 agonist), and placebo, respectively, were administered intravenously preceded by a control period. Terbutaline, 0.5 mg intravenously, was followed by a significant decrease of sigmoid motility from 4.3 +/- 1.5 (SEM) kPa X min to 2.9 +/- 1.0 kPa X min (P less than 0.01) and of rectal motility from 4.3 +/- 1.3 to 2.4 +/- 0.7 kPa X min (P less than 0.05). After placebo a slight, but not significant, increase of contractile activity was seen compared to the initial control period. The effects of prenalterol, 1.0 and 4.0 mg intravenously, on motility did not differ from that of placebo infusion. Both drugs caused a dose-dependent increase of systolic blood pressure and of heart rate. The study shows that beta-2-adrenoceptor stimulation decreases rectosigmoid colonic pressure in man, while effects of beta-1 stimulation on motility index do not differ from that of placebo.