Changes in femoral vein pH, lactate, glucose and potassium were studied in a double-blind randomized, short-term, dynamic cycle ergometry exercise test on six healthy male subjects after administration of non-selective (timolol), beta-1-selective (atenolol) beta blocker or placebo. The exercise intensity was increased in steps of 200 kpm/min every 2 min until exhaustion. During submaximal exercise, potassium concentrations in blood from the exercising leg muscles increased progressively with increasing exercise intensity, and was significantly higher for any given exercise level following timolol as compared to placebo administration. The potassium concentrations following atenolol were in-between those of timolol and placebo. Despite reduced working capacity after non-selective beta blockade, almost identical potassium concentrations were reached at exhaustion irrespective of treatment regimens (placebo: 6.3, range 5.8-6.8 mmol/l; atenolol: 6.5, range 6.1-7.3 mmol/l and timolol: 6.4, range 6.2-6.8 mmol/l). The increase in s-lactate concentrations was similar across all treatments, and rose in proportion to the increase in the exercise intensity. A biphasic increase in lactate was observed with identical breaking points (anaerobic threshold) irrespective of treatment regimens. There was no difference in glucose concentrations between the treatment regimens. The marked increase in serum potassium during maximal exercise coincides with leg muscle fatigue and may, by its effect on the muscle cell membrane potential, limit the maximal working capacity following beta blockers. The rise in serum potassium may curtail the use of maximal exercise test as an index of cardiac performance in healthy young subjects.