β-Adrenoceptor blockers are widely used clinically and can be classified as nonselective (β1 and β2) or selective (β1). Impairment of exercise performance is a well-known side effect of this group of drugs. This paper reviews mechanisms that could potentially be responsible for this impairment. In addition to cardiovascular and metabolic effects, β -blockade inhibits Na+–K+ ATPase pumps controlling ion movement between muscle and plasma and thus may contribute to muscle fatigue through this mechanism. To investigate the relationship between the change in plasma [K+] and exercise performance, we studied healthy male subjects taking propranolol. Eight subjects performed maximal incremental cycle ergometer exercise tests during control (no drug), low dose (LD) (40 mg daily), and high dose (HD) (265 ± 4.3 (SE) mg daily) of propranolol. The control plasma [K+] (5.8 ± 0.12 mequiv./L) during exercise was significantly lower than either the LD (6.4 ± 0.05 mequiv./L) or HD (6.1 ± 0.16 mequiv./L) values. There was no significant difference between plasma [K+] for the LD and HD of propranolol. However, maximum oxygen uptake was reduced only while taking the HD of propranolol. Six of the subjects also performed three 30-s bouts of high intensity exercise on an isokinetic cycle ergometer while taking the LD and HD of propranolol. There was no significant difference between doses for the increase in plasma [K+] (LD, 7.8 ± 0.35 mequiv./L vs. HD, 7.6 ± 0.36 mequiv./L) during exercise. However, exercise performance was significantly reduced during HD compared with LD. These results suggest that the increases in plasma [K+] with propranolol did not play a direct significant role in the reduced performance observed during the HD.Key words: exercise, potassium, performance, lactate.