Controversy exists over whether defects in electrolyte transport exist in erythrocytes from cystic fibrosis (CF) patients. We hypothesized that differences in ion regulation in CF would affect skeletal muscle performance during intense exercise. Seven well-nourished CF patients were compared with seven healthy age-matched control subjects. Skeletal muscle performance was assessed during a 30-s sprint on an isokinetic cycle ergometer. Ion regulation was evaluated from arterialized venous blood sampled at rest, at peak exercise, and after 5 min of recovery. There was no difference in sprint performance between the CF (total work, 93.7 ± 30.02% predicted; endurance, 30.6 ± 9.93% decline) and control (109.7 ± 19.48%; 35.6 ± 14.76%) groups. The changes in plasma and erythrocyte ions and blood gases did not differ between the groups. There was a suggestion that the CF group may have had an inadequate ventilatory response to the metabolic challenge of short-term maximal exercise. The contribution of decreases in the strong ion difference to increases in plasma hydrogen ion concentrations was less in the CF group. This may be due to alterations in ionic regulation in CF, but the influence of inadequate arterialization of the blood samples could not be ruled out.Key words: isokinetic cycle ergometer, ion movement, hydrogen ion, cystic fibrosis.