The objective of this study was to compare caloric expenditure and substrate utilization during exercise begun at different time intervals following a standard test meal or in the fasted state. Eight physically fit women (aged 21-27 years) participated in four separate exercise trials. In three trials, the subjects consumed a 940-kcal meal following an overnight fast and began exercising either 30, 60, or 90 min after the meal. In the other trial, the subjects did not consume any breakfast prior to exercising. Energy expenditure and substrate utilization were determined by indirect calorimetry during the last 23 min of a 30-min run on a treadmill at an average work load of 62% VO2max. There were no significant differences among trials when comparing the total caloric expenditures (range: 215-219 kcal). However, the subjects oxidized significantly more fat (94.3 kcal) when they exercised on an empty stomach than when they exercised 60 or 90 min after the meal (71.6 and 68.8 kcal, respectively) (P less than 0.05). It was concluded that consumption of a meal prior to exercise does not increase the energy cost of the activity for physically fit women, but it does disrupt the pattern of substrate utilization, reducing the contribution of fat as an energy source.