We determined the interaction of diet and training on metabolic adaptations in skeletal muscle and liver, and the consequences of these adaptations for endurance. Eighty rats performed a baseline treadmill run to exhaustion at 16 m min-1 (RUN1) and were then divided into two groups and given one of two diets: high carbohydrate (CHO) or high fat (FAT). Each dietary group was then divided into one of four subgroups: sedentary control that performed no training (NT); low-intensity running (8 m min-1; LOW) and two groups who trained at their maximal voluntary running speed without electrical stimulation (28 m min-1; VMAX). Training volume was identical for LOW and VMAX (1000 m session-1) and animals ran 4 days week-1 for 8 weeks. To assess the interaction of the higher intensity exercise with diet, a second endurance test (RUN2) was undertaken after 6 weeks at either 16 m min-1 or 28 m min-1. The NT group ran for a longer duration (increase of 77 %) after FAT than CHO (239 ± 28 vs. 135 ± 30 min, P < 0.05) at 16 m min-1. There were no differences in RUN2 for the LOW group when rats ran at 16 m min-1 (454 ± 86 vs. 427 ± 75 min for CHO and FAT groups, respectively), but rats in the VMAX group fed FAT ran longer than rats fed CHO at 28 m min-1 (100 ± 28 vs. 58 ± 11 min, respectively, P < 0.05). FAT increased the activities of the enzymes citrate synthase, β-hydroxyacyl-CoA dehydrogenase and carnitine palmitoyl-transferase compared to CHO (P < 0.01), but there was no systematic effect of training. We conclude: (1) there was no additive effect of a high-fat diet on endurance performance when rats performed low-intensity training; (2) running performance at 28 m min-1 was only enhanced by a high-fat diet after more intense training; (3) diet-induced and training-induced adaptations that increase exercise capacity may be under independent control. Experimental Physiology (2001) 86.4, 499-508.