Six healthy male subjects exercised after an overnight fast for a fixed 3 min period at a workload equivalent to 100% of their maximal oxygen uptake (VO2max) on 3 separate occasions. The first test took place after subjects had consumed a mixed diet (43 +/- 3% carbohydrate (CHO), 41 +/- 5% fat and 16 +/- 3% protein) for 3 days, and was followed 2 h later by prolonged cycling to exhaustion at 77 +/- 3% VO2max to deplete muscle glycogen stores. Following this, subjects consumed a low CHO diet (4 +/- 1% CHO, 63 +/- 5% fat and 33 +/- 6% protein) for the remainder of the day and for the subsequent 2 days; on the morning of the next day a second high intensity test took place. Finally subjects followed a 3 day high CHO diet (73 +/- 7% CHO, 17 +/- 6% fat and 10 +/- 1% protein) before their last test. Acid-base status and selected metabolites were measured on arterialized-venous blood at rest prior to exercise and at intervals for 15 min following exercise. Prior to exercise, plasma pH and blood lactate concentration were higher (p less than 0.05) after the high CHO diet when compared with the low CHO diet. Pre-exercise plasma bicarbonate, blood PCO2 and blood base excess were all higher after the high (p less than 0.001, p less than 0.01, p less than 0.01 respectively) and normal (p less than 0.05, p less than 0.05, p less than 0.05 respectively) CHO diets when compared with the low CHO diet.(ABSTRACT TRUNCATED AT 250 WORDS)