The effects of topical timolol vs betaxolol on cardiopulmonary exercise performance were studied in a randomised double-masked fashion in 12 healthy male volunteers. Cardiopulmonary parameters were evaluated at the anaerobic threshold and at peak exercise. Intraocular pressure was determined before and after treatment by applanation tonometry. No differences were found in aerobic or peak exercise capacity. Maximal heart rate was slightly lower (P less than 0.05) following treatment with timolol compared with betaxolol. However, a correspondingly higher oxygen pulse (oxygen uptake/heart rate) compensated for this reduction and resulted in no difference in peak performance. At physiological work levels, it was not possible to demonstrate any influence of topical, selective or non-selective, beta-adrenergic blockade on cardiopulmonary exercise performance in these healthy volunteers.