Muscular exercise is the most common stress imposed on the cardiovascular system and, in hypertensive patients, it causes an exaggerated increase in the already elevated blood pressure. The evaluation of any antihypertensive drug must, therefore, include an investigation of its effects on the haemodynamic response to exercise. For this reason the effects of celiprolol and enalapril were studied in hypertensive patients, both at rest and during an exercise stress test performed on a bicycle ergometer. The haemodynamic changes observed were very similar at rest: both drugs consistently reduced blood pressure without impairing either myocardial geometry or function. The only between drug difference found at rest was slight bradycardia with celiprolol, whereas heart rate was unaffected by enalapril. By contrast, there was a marked difference in the effect on the blood pressure increase caused by muscular exercise: 24 h after dosing, celiprolol continued to attenuate the hypertensive response to exercise while enalapril failed to show any significant antihypertensive effect possibly because after this time there was a reduction in angiotensin converting enzyme inhibiting activity. Thus, celiprolol and enalapril are both effective once-a-day antihypertensive agents, but celiprolol provides a longer lasting protection from hypertensive peaks caused by exercise.