These studies investigated the effects of 2 weeks of either a high-fat (HIGH-FAT: 70% fat, 7% CHO) or a high-carbohydrate (HIGH-CHO: 74% CHO, 12% fat) diet on exercise performance in trained cyclists (n = 5) during consecutive periods of cycle exercise including a Wingate test of muscle power, cycle exercise to exhaustion at 85% of peak power output [90% maximal oxygen uptake (VO2max), high-intensity exercise (HIE)] and 50% of peak power output [60% VO2max, moderate intensity exercise (MIE)]. Exercise time to exhaustion during HIE was not significantly different between trials: nor were the rates of muscle glycogen utilization during HIE different between trials, although starting muscle glycogen content was lower [68.1 (SEM 3.9) vs 120.6 (SEM 3.8) mmol.kg-1 wet mass, P < 0.01] after the HIGH-FAT diet. Despite a lower muscle glycogen content at the onset of MIE [32 (SEM 7) vs 73 (SEM 6) mmol.kg-1 wet mass, HIGH-FAT vs HIGH-CHO, P < 0.01], exercise time to exhaustion during subsequent MIE was significantly longer after the HIGH-FAT diet [79.7 (SEM 7.6) vs 42.5 (SEM 6.8) min, HIGH-FAT vs HIGH-CHO, P < 0.01]. Enhanced endurance during MIE after the HIGH-FAT diet was associated with a lower respiratory exchange ratio [0.87 (SEM 0.03) vs (SEM 0.02), P < 0.05], and a decreased rate of carbohydrate oxidation [1.41 (SEM 0.70) vs 2.23 (SEM 0.40) g CHO.min-1, P < 0.05].(ABSTRACT TRUNCATED AT 250 WORDS)