Active muscle releases K+, and the plasma K+ concentration is consequently raised during exercise. K+ is removed by the Na,K pump, and training may influence the number of pumps. The plasma K+ concentration was therefore studied in five endurance-trained (ET) and six sprint-trained (ST) subjects during and after 1 min of exhausting treadmill running. Non-exhausting bouts of exercise at either lower speed or of shorter duration were also carried out. Blood samples were taken from a catheter in the femoral vein before and at frequent intervals after exercise. The pre-exercise venous plasma [K+] was (mean +/- SEM) 3.68 +/- 0.10 mmol l-1 (ET) and 3.88 +/- 0.06 mmol l-1 (ST). One minute of exhausting exercise was sustained at 5.27 +/- 0.08 m s-1 (ET) and 5.59 +/- 0.06 m s-1 (ST) and caused the plasma K+ concentration to rise by 4.4 +/- 0.3 (ET) and 4.7 +/- 0.3 mmol l-1 (ST; ns) respectively. Three minutes after exercise the K+ concentration was 0.48 +/- 0.08 mmol l-1 (ST) and 0.50 +/- 0.07 mmol l-1 (ST) below the pre-exercise value. During the following 6 min of recovery, the value was unchanged for the ET subjects, while a 0.32 +/- 0.06 mmol l-1 rise was seen for the ST subjects. Exercise at reduced intensity or of reduced duration resulted in smaller changes in the K+ concentration both during exercise and in the post-exercise recovery, and for each subject the lowest post-exercise K+ concentration was therefore inversely related to the peak K+ concentration during exercise.(ABSTRACT TRUNCATED AT 250 WORDS)