1. Many previous reports have shown that the ratio of total body potassium to fat-free tissue mass is, on average, higher in men than in women.

2. In an attempt to explain this finding we have re-examined our own data and data taken from the literature. Our own data comprise measurements on 333 healthy people (196 men and 137 women). In all subjects we measured total body potassium and estimated fat-free mass from body weight and skinfold thickness; in 91 of them (62 men and 29 women) we measured, in addition, total body nitrogen.

3. We have used the statistical technique of factor analysis to obtain unbiased estimates of the relationships between the three quantities measured in the smaller group. The validity of the relationships is supported by the results from the larger group and by data from the literature.

4. The average values of total body potassium and fat-free mass are greater in men than in women, but the relationship between the two variables is identical in men and women. The relationship has a positive intercept on the axis of fat-free mass, so that the ratio of total body potassium to fat-free mass increases with fat-free mass. The higher ratio in men compared with women can therefore be explained by a difference in size alone.

5. The relationship between total body potassium and total body nitrogen has a negligible intercept. Therefore the ratio of total body potassium to total body nitrogen does not change with size.

6. These findings suggest that, on average, the fat-free mass is made up of a fixed component of approximately 9 kg of tissue containing no potassium or nitrogen, and a variable component with a potassium to nitrogen ratio of about 1.81 mmol/g.

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