Four subjects exercised at two levels of power output, approximating to 30% and 60% of their maximum O2 uptake, on three occasions: after a 24-h fast, after a 12-h fast, and 1 h after a 3000 Cal mixed meal. Measurements during exercise did not show any consistent effect on O2 uptake or heart rate. CO2 output, the respiratory exchange ratio, and ventilation were increased following the meal. Changes in blood lactate and the lactate/pyruvate ratio were variable and small. The increase in blood glycerol with exercise was least following the meal, indicating a lower level of fat mobilization; blood free fatty acids were lower. Blood glucose fell (from high resting levels) in three subjects following the meal, suggesting an increased usage of blood-borne glucose. The physiological effects of a meal on exercise performance in healthy subjects appear to be due mainly to inhibition of free fatty acid mobilization. This leads to an increased usage of carbohydrate energy sources, higher respiratory exchange ratio, and CO2 output, in turn leading to higher ventilation. No cardiovascular effects were detected.