Six well-trained men were studied while performing a maximal bicycle exercise. The seven experiments included in this study were randomized in a double-blind cross-over fashion. On each occasion the subjects were given either placebo or 40, 80, or 160 mg propranolol (non-selective blockade) or 25,50, or 100 mg atenolol (beta 1-selective blockade). After completion of the study each subject had performed once under each of the seven treatments. Heart rate, maximal oxygen uptake (Vo2max), blood lactate and performance time to exhaustion were measured. A muscle biopsy from vastus lateralis was taken at exhaustion after placebo, 80 mg propranolol and 50 mg atenolol trials, for analysis of ATP, creatine phosphate (CP), glucose-6-phosphate (G-6-P), glucose and lactate. The performance time was reduced (P less than 0.05-0.001) with both blockers compared to placebo. At an equal heart rate reduction, Vo2max was equally reduced by both blockers. Performance time, on the other hand, was reduced to a greater extent (P less than 0.05) with propranolol. ATP and CP levels were decreased (P less than 0.05) by both drugs. G-6-P, however, was lower (P less than 0.05) with propranolol than with either placebo or atenolol. No difference was observed between placebo and atenolol. In conclusion, both beta1-selective and non-selective blockade reduced short-term maximal exercise capacity. The major limiting factor seems to be the reduction in oxygen transport. The finding that at an equivalent reduction in Vo2max propranolol reduced performance time to a greater extent than atenolol suggests that beta 2-blockade may reduce performance by mechanisms additional to those that affect oxygen transport.(ABSTRACT TRUNCATED AT 250 WORDS)