The acute and chronic circulatory effects of a new beta adrenergic blocking drug, dl-1-(2,5-dichlorophenoxy)-3-tertiary-butylamino-2-propanol hydrochloride, GYKI-41099, have been assessed in patients with hyperkinetic circulation/or mild hypertension. The acute effects of GYKI-41099 2.5, 5 and 10 mg have been compared with those of propranolol 10, 20 and 40 mg using a cross-over design in 18 patients with elevated cardiac index. Each dose of both drugs produced a significant reduction in sitting and exercise heart rate. The potency of GYKI-41099 compared with propranolol proved to be 3:1. 13 patients with hyperkinetic circulation on GYKI-41099 5-15 mg daily for 2 months exhibited a significant reduction in resting heart rate and cardiac index. In 13 patients with mild hypertension the drug exerted marked antihypertensive effect. Side effects were minimal and short-lived.