1 The effect of oral doses of the pl-selective adrenoceptor antagonist atenolol (50 mg), the non-selective antagonist propranolol (40 mg) and placebo was investigated during exercise in a crossover comparison in six healthy but untrained subjects. Descriptors of ventilation, respiratory gas exchange, and arterialized blood lactate and glucose were obtained during steady state bicycle ergometric exercise at 20% and 60% of the subjects' previously determined maximal oxygen uptake (Vo2 max). 2 At these work intensities, the previously reported increase of respiratory exchange ratio (RER) during non-selective 3-adrenoceptor blockade was found to be trivial (placebo = 0.96 ± 0.03 s.e. mean; propranolol = 0.97 + 0.01; atenolol = 0.97 + 0.04; 60% Vo2 max, 10 min exercise) and only present during the early minutes of effort.Oxygen uptake and carbon dioxide production did not differ between treatments. 3 Both drugs produced highly significant falls in peak expiratory flow (PEF) rates and tidal volume (VT) which were compensated by an increase in respiratory rate. PEF, 60% Vo2 max: placebo = 3.8 + 0.3 l/s; propranolol 3.6 + 0.3 1/s (P < 0.03); atenolol 3.1 + 0.3 1/s (P < 0.01). VT, 60% Vo2 max: placebo 2.0 + 0.1 1; propranolol 1.8 + 0.2 1 (P < 0.05); atenolol 1.7 ± 0.1 1 (P < 0.01). 4 Arterialized lactate was significantly elevated during work at 20% and 60% Vo2 max, but rose progressively at the 60% Vo2 max load. Ventilation, oxygen uptake and ventilatory equivalent for carbon dioxide also rose progressively at this workload. Ventilatory equivalent for oxygen showed no significant rise. There were no treatment differences, either at the aerobic or anaerobic workloads. During steady state exercise, previously described determinants of anaerobic threshold (RER, 02 ventilatory equivalent) appear not to be valid. 5 We conclude that normoglycaemic aerobic or anaerobic work is associated with a I3l-adrenoceptor response which reduces airway resistance. Despite antagonism of lipolysis and hepatic glycogenolysis, ,B-adrenoceptor blockade does not appear to alter the pattern of substrate utilization.