We investigated the ionic changes in arterial (a) and femoral venous (fv) blood that accompany muscle fatigue with repeated maximal exercise. Measurements were made on separated plasma and hemolysed whole blood to quantify the relative contributions of plasma and erythrocytes to this acid-base challenge. Five healthy males performed four 30-s bouts of maximal isokinetic cycling exercise, with 4 min of rest between bouts, and recovery was followed for 90 min. In whole blood, maximal increases in [K+]a amounted to 10 +/- 2.0 meq/l and in [K+]fv to 7 +/- 4.3 meq/l and occurred at the end of bout 2. Whole blood lactate concentration ([Lac-]) peaked at 15.3 +/- 1.39 ([Lac-]a) and 16.7 +/- 1.59 meq/l ([Lac-]fv) at the end of bout 4. In plasma, peak [Lac-]a and [Lac-]fv were both 21 meq/l at the end of bout 4. Plasma [H+]a increased from 36 +/- 1.0 neq/l at rest to 44 +/- 2.9 neq/l at the end of the first bout of exercise; 80% of this increase was due to a 2.9 meq/l decrease in arterial strong ion difference ([SID]), and 20% was due to an increase in plasma protein ([Atot]a); a reduction in arterial PCO2 to 29 mmHg had an alkalinizing effect. In contrast, plasma [H+]fv increased from 39 +/- 0.5 neq/l at rest to 93 +/- 4.1 neq/l, with an increase in PfvCO2 to 97 +/- 7 mmHg contributing 75%, a decrease in [SID]fv 15%, and an increase in [Atot]fv 10% to the increase in [H+]fv. In later exercise bouts, the relative contributions of [SID]a, [Atot]a, and arterial PCO2 to plasma [H+]a were similar, but the contribution of [SID]fv to [H+]fv increased and that of femoral venous PCO2 decreased, with the contribution of [Atot]fv remaining unchanged (8-12%). During exercise and recovery, the changes in both arterial and femoral venous PCO2 and [K+] were more rapid than changes in [Lac-], and the time course of whole blood [K+] was slower than that of plasma [K+]. Erythrocytes may play an important role in regulating plasma [Lac-] and [K+] with intense exercise.