Six healthy subjects walked 37 km per day for four consecutive days on two occasions one month apart; on one walk, subjects consumed a high carbohydrate (CHO) diet (85 +/- 1% CHO, Mean +/- SE) and on the other walk an isocaloric low CHO diet (2 +/- 0% CHO) was consumed. Subjects were fasted each day until after the completion of the walk. Blood samples were obtained at rest prior to exercise and after completion of each of three laps of 12.3 km. Exercise intensity corresponded to approximately 17% of VO2max. The first day of each walk demonstrated that the pattern of substrate mobilisation in response to this type of exercise is highly reproducible, there being no difference in any of the parameters measured between the two walks. Circulating glucose, lactate, insulin and triglyceride levels remained essentially unchanged; alanine fell progressively and glycerol, free fatty acid (FFA) and 3-hydroxybutyrate (BHB) rose progressively. After the first day there was a general tendency for the blood glucose concentration to decline as exercise progressed; by the end of the walk on Day 2, blood glucose was lower on the low CHO diet than on the high CHO diet. On Day 4 plasma insulin was higher (p less than 0.05) on the high CHO diet than on the low CHO diet and declined progressively on both diets. Blood lactate and alanine concentrations were generally higher at rest on the high CHO diet, but fell so that no differences existed by the end of exercise.(ABSTRACT TRUNCATED AT 250 WORDS)