The effect of beta-adrenoceptor antagonists on the respiratory response to carbon dioxide rebreathing was studied in eight normal subjects. Propranolol, atenolol, metoprolol, and placebo were given in random, double-blind fashion. Subjects were studied before each treatment period, after one dose, and after eight days of treatment with each drug. A rebreathing method was used to produce progressive hypercapnia and the respiratory response was assessed by measuring minute ventilation and maximum rate of change of inspiratory mouth pressure. Beta-blockade was assessed by the reduction in heart rate during steady state exercise on a cycle ergometer. There was no change in the respiratory response to carbon dioxide after a single dose or eight days treatment of any drug. All three active drugs produced a significant reduction in exercise heart rate. The forced expiratory volume in one second was not altered by any of the drugs.