A technique based on a completely metabolizable liquid diet was developed rapidly to achieve weight stability (+/- 50 g/day) in experimental subjects. Weight-stabilizing caloric intake was then used as an estimate of total daily energy expenditure (24 h EE) in a study of 10 ambulatory obese subjects before and after weight loss. In five control subjects 24 h EE fell by 928 +/- 121 kcal/day (3885 +/- 507 kJ/day) or 29 +/- 3 percent after a 22.6 +/- 1.2 percent weight loss achieved on a three month 700 kcal/day (2930 kJ/day) diet. To determine how much of this decrease was due to the reduced thermic effect of exercise (TEE) at the lower body weight, the remaining five subjects underwent progressive exogenous weight replacement during dieting to compensate exactly for their 21.0 +/- 5.2 percent weight loss. In the latter group 24 h EE fell by only 392 +/- 192 kcal/day (1641 +/- 804 kJ/day) or 12 +/- 5 percent (P less than 0.001). These data, along with indirect calorimetric measurements of resting metabolic rate (RMR), permitted the decrease in 24 h EE with weight loss to be divided into its component parts. Decreases in RMR, TEE and the thermic effect of food accounted for 30 percent, 52 percent and 18 percent respectively of the fall in 24 h EE. A reduction in TEE appears to make a greater contribution to the enhanced energy efficiency of ambulatory reduced-obese subjects than has been suggested by studies based on methods requiring greater subject confinement.