1. The effects on heart rate, blood pressure and pulmonary function of single oral doses of celiprolol hydrochloride (400 mg), and propranolol (40 mg) were compared with placebo in 12 healthy volunteers, in a double-blind three-period crossover study. 2. Celiprolol had no effect on heart rate while propranolol caused a significant reduction compared with placebo. Systolic blood pressure was reduced by propranolol but not celiprolol, whereas standing diastolic blood pressure was lowered by both drugs. 3. The maximal expiratory flow at 50% vital capacity (MEF.50), was significantly lower after propranolol compared with placebo and celiprolol. Celiprolol had no effect on the flow-volume loop parameters. 4. Effective pulmonary blood flow was significantly increased by celiprolol, but reduced by propranolol. 5. A high incidence of subjective side-effects were experienced on celiprolol (10/12; particularly unpleasant in 5). Side-effects were experienced to a lesser extent on placebo (8/12). Only one volunteer experienced a side-effect on propranolol. 6. Oral celiprolol exerts its hypotensive effect by vasodilatation without reflex tachycardia. It does not cause airways obstruction in healthy subjects.