The relationship between resting metabolic rate and different parameters of body size was investigated among 28 female volunteers in the age group of 20--30 years. The resting metabolic rate of the subjects was determined indirectly by measuring the oxygen consumption in a closed circuit, in which the oxygen concentration was stabilised. The fat percentage of the body was determined by densitometry. The population was divided into two groups: the obese, with an average fat percentage of 33.6 and the normal-weight with an average fat percentage of 20.4. Mean values for the resting metabolic rate were 1550 kcal/24 h (6.488 MJ/24 h) for the obese and 1421 kcal/24 h (5.948 MJ/24 h) for the normal-weight group. The resting metabolic rate per kg body weight was lower in the obese than in the normal-weight persons. However, expressed per kg fat-free body mass, energy expenditure under resting conditions in the obese was higher than in the normal-weight. No single body parameter seems to be suitable in the explantation of RMR in women with substantially different fat content. The best prediction of resting metabolic rate in this population of obese and normal-weight women is obtained when both fat-free mass and fat mass are used as independent variables in a linear regression equation.