Abstract 1. 1. Patients with bums covering up to one third of the body surface treated in a relatively cool environment lost at least 10% of their admission body weight during a period of up to 5 weeks in hospital. Patients with more extensive burns may lose at least 20% of their admission body weight during this time. 2. 2. Estimations of 24-h exchangeable potassium content in the same patients reveal losses of body potassium, expressed as a percentage of the expected normal value which are at least as great as the percentage losses of body weight. Some of this potassium loss will be derived from the catabolism of lean tissue, and corresponding excess nitrogen losses were observed. 3. 3. The pattern of zinc and creatine excretion indicates that muscle tissue may be the main constituent of the lean body tissue which is catabolised. 4. 4. In patients with bums covering up to one third of the body surface the presumed muscle tissue catabolism accounts for most of the observed loss of body weight. 5. 5. In patients with very extensive bums muscle tissue and fat contribute about equally to the observed loss of body weight. In these patients the excess potassium excretion may be derived partly from tissue catabolism and partly from a reduction in the potassium content of living cells.