1. The food intake of sixty-four infantry recruits was measured at six centres during 3 weeks of initial training. The daily energy expenditure was measured in thirty-five of these men.
2. The mean daily consumption of the sixty-four subjects provided them with 3850 kcal (16110 kJ); the energy expenditure of the thirty-five subjects averaged 3750 kcal (15690 kJ).
3. Serial auto- and cross-correlations of intake and expenditure were very small and there was no significant relationship between food intake and energy expenditure on the same day.
4. The intakes and expenditures of different subjects at the same centre were not independent.
5. There was a significant relationship between intake and expenditure for the whole period of the survey when results for all subjects were included. In three centres the correlation was high, +0.788 (P < 0.001), but was only + 0.083 (P < 0.5) in the remaining three centres.
6. There was a positive but not significant correlation between body-weight and the average food intake of 6 d.
7. There was a negative correlation between body-weight and calorie balance.
8. Weight change and calorie balance over 1 week were related, the correlation averaging 0.40. There was a correlation of 0.32 between daily weight changes and calorie balance. A rather small amount of variation in calorie balance can be explained by contemporary changes in weight.