1. Ten untrained young men ingested a carbohydrate-rich diet (65 energy percent (E%) carbohydrate, T-CHO) and ten similar subjects a fat-rich diet (62 E% fat, T-FAT) while endurance training was performed 3-4 times a week for 7 weeks. For another 8th week of training both groups ingested the carbohydrate-rich diet (T-CHO and T-FAT/CHO). 2. Maximal oxygen uptake increased by 11% (P <0.05) in both groups after 7 and 8 weeks. Time to exhaustion at 81% of pre-training maximal oxygen uptake increased significantly from a mean (+/- S.E.M.) of 35 +/- 4 min to 102 +/- 5 and 65 +/- 7 min in T-CHO and T-FAT, respectively, after 7 weeks (P <0.05, T-CHO vs. T-FAT). After 8 weeks, endurance remained unchanged in T-CHO but increased (P <0.05) to 77 +/- 9 min in T-FAT/CHO which, however, was still less (P <0.05) than in T-CHO. 3. Muscle glycogen breakdown rate during exercise was halved by endurance training equally in both T-CHO and T-FAT after 7 and 8 weeks, and muscle glycogen stores at exhaustion were not depleted in any group. 4. During exercise after 7 weeks, the respiratory exchange ratio (RER) was unchanged in T-CHO (0.88 +/- 0.01) compared with pre-training but decreased (P <0.05) to 0.82 +/- 0.02 in T-FAT. After 8 weeks, RER in both T-CHO and T-FAT/CHO was approximately 0.87. 5. During exercise, plasma noradrenaline concentration and heart rate were higher in T-FAT than in T-CHO both at 7 and at 8 weeks. 6. It is concluded that ingesting a fat-rich diet during an endurance training programme is detrimental to improvement in endurance. This is not due to a simple lack of carbohydrate fuel, but rather to suboptimal adaptations that are not remedied by short-term increased carbohydrate availability. Furthermore, the study suggests that the decrease in RER usually seen after training when exercising at the same absolute intensity as before training can be prevented by a carbohydrate-rich diet.