Effects of propranolol and metoprolol on sigmoid colonic motility were studied in 12 healthy volunteers in a double-blind randomized fashion. Colonic pressure was recorded 15-18 cm from anus and contractile activity quantified for periods of 25 min. On separate days propranolol, metoprolol, and placebo, respectively, was administered intravenously preceded by a control period. After propranolol, 10 mg intravenously, pressure activity increased significantly from 3.8 +/- 1.1 (SEM) kPa X min (28 +/- 8 mm Hg X min) to 5.9 +/- 1.0 kPa X min (44 +/- 8 mm Hg X min) (P less than 0.001). Also, after propranolol, 5 mg intravenously, the pressure activity was increased (P less than 0.05). After metoprolol, 10 mg intravenously, contractile activity increased from 4.3 +/- 0.9 kPa X min (32 +/- 7 mm Hg X min) to 6.1 +/- 1.0 kPa X min (46 +/- 8 mm Hg X min) (P less than 0.01). The two drugs caused equipotent reduction of heart rate. After placebo, no effect on sigmoid pressure or heart rate was observed. The study shows that unselective (propranolol) and beta 1-selective (metoprolol) beta-blocking drugs enhance distal colonic pressure in man. Colonic motility seems to be under sympathetic beta-adrenergic influence even under fairly unstrained conditions.