SUMMARYTotal body potassium has been measured in chromatin positive males (Klinefelter's syndrome) and males with a 47,XYY karyotype. In patients with an extra X chromosome the level of potassium was very significantly less than in normal males. Its concentration referred to lean body mass, estimated from the patient's height and weight, was also greatly reduced and not significantly different from values found in normal women.In 47, XYY males individual values were low but there was no reduction in the mean value for the group if allowance was made for the obesity of some XYY subjects. The significance of these findings, however, is difficult to assess as ther height frequently exceeded that of the controls on which the predicted potassium values were based.From these findings it would seem taht when characteristics which are normally associated with the female sex occur in males, as in Klinefelter's syndrome, there is also a reduction in body potassium either in the total content or the lean tissue concentration, or in both.By comparison, 47, XYY males appear to be more normal in these respects but further normal data is requird in order to interpret the results.The possibility that abnormal androgen production in the chromatin positive men may be influencing the potassium levels is the subject of further investigation.