Adrenergic mechanisms modulate exercise-induced changes in blood serum K+ concentration ([K+]). Impairment of these same mechanisms may be associated with bronchial hyper-reactivity. If this is accurate, asthmatic subjects should show disturbed K+ regulation during exercise. We measured [K+] and FEV1 in 13 healthy control and in 13 asthmatic subjects pre-exercise, at peak exercise (within 1 min of stopping exercise), and 10 min postexercise. This was done on 2 separate days, one with and one without bronchodilator (BD) pretreatment. Both groups were equally fit, exercising to the same O2 consumption and heart rate. Resting [K+] was normal for both groups (two-day averages were 4.00 +/- 0.07 and 4.09 +/- 0.07 mmol/L, mean +/- SEM, in control and asthmatic subjects, respectively). Without BD pretreatment, at peak exercise, [K+] in control subjects rose by 0.56 +/- 0.08 compared with 0.96 +/- 0.09 in asthmatics (p less than 0.01). After exercise, [K+] returned to baseline (4.12 +/- 0.08) in control subjects but remained elevated in asthmatics (4.60 +/- 0.12, p less than 0.01). Although FEV1 was unchanged in control subjects, in asthmatics it fell after exercise (p less than 0.01). With BD pretreatment: peak exercise [K+] increased by 0.55 +/- 0.09 in control subjects, and by 0.49 +/- 0.01 in asthmatics (p less than 0.01). By 10 min postexercise, it returned to baseline in both groups (4.15 +/- 0.11 for control subjects and 4.32 +/- 0.07 for asthmatics). The asthma group's fall in FEV1 was also abolished. These data indicate that postexercise K+ remains elevated in asthmatics, supporting the suggestion that their adrenergic function is impaired.