Seven healthy male subjects exercised to exhaustion at a workload equivalent to 100% of their maximal oxygen uptake (VO2max) on 3 separate occasions. Each high intensity exercise test was performed on an electrically braked cycle ergometer; the first took place after a normal diet (46 +/- 8% carbohydrate (CHO), 41 +/- 7% fat and 13 +/- 3% protein); the second after 3 days of a low CHO diet (7 +/- 3% CHO, 64 +/- 5% fat and 29 +/- 4% protein) and the third after 3 days of a high CHO diet (76 +/- 6% CHO, 14 +/- 5% fat and 10 +/- 2% protein). Acid-base status and selected metabolites were measured on arterialized venous blood at rest prior to exercise and during the post-exercise period. Plasma urea concentration and urine total acidity were measured on each day of the experiment. Exercise time to exhaustion was longer after the normal (p less than 0.05) and high (p less than 0.01) CHO diets compared with the low CHO diet. Pre-exercise plasma bicarbonate concentration and blood PCO2 were higher after the high CHO diet when compared with the normal (p = 0.05, p less than 0.05 respectively) and low CHO conditions (p less than 0.05, p less than 0.05 respectively). Pre-exercise bicarbonate was also higher after the normal CHO diet when compared with the low CHO diet (p less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)