Bopindolol is a potent and specific beta-adrenoceptor antagonist with partial agonist activity. In animal experiments it blocks both beta 1- and beta 2-adrenoceptors and possesses a long duration of action. In the present study in healthy volunteers bopindolol was about ten times more potent than pindolol in reducing isoprenaline-induced and exercise-induced tachycardia. In experiments on exercise-induced tachycardia an oral dose of 2 mg produced a near maximum reduction of exercise heart rate, occurring within 2 to 3 h of administration. With higher doses (up to 12 mg) the maximum effect was reached earlier (between 1 and 2 h). The long duration of action of bopindolol observed in animal studies was confirmed in man. Twenty-four hours after 4 and 10 mg bopindolol more than 2/3 of the maximum effect was still present. After 48 h 38% of the maximum effect of 4 mg and 50% of that of 12 mg remained. Even at 72 and 96 h exercise-induced tachycardia was still significantly lowered after both doses of the drug. When bopindolol was administered once daily for 5 days there was a slight increase in the maximum reduction of exercise-induced tachycardia during treatment with 1 mg/day but not with 4 mg/day, which produced a near maximum effect.