1. Total body potassium ( 40 K method) and total body water and exchangeable sodium (both by isotope dilution) were determined in 26 boys, aged 5-17 years, with muscular dystrophy. Total body potassium values were compared with measurements in a large series of normal boys on the basis of height. 2. Total body potassium was reduced even in the youngest patients and was only slightly higher in the older boys, despite their considerably greater height. Exchangeable sodium increased with increasing height in a way similar to that of normal boys. Total body water was also reduced but increased with growth, although to a lesser extent than expected for normal boys. The total body water measurements indicated that many of the affected boys were very obese, despite an apparently normal body weight. 3. An intravenous bolus of 22 Na distributed at a similar rate in boys with muscular dystrophy to that in normal males. 4. In relation to the predicted values, total body potassium and 24 h urinary creatinine excretion of the affected boys both declined at a rate of 4% per year.