Alterations occur in human muscle electrolyte and water composition in response to infection. There appear to be at least two basic mechanisms; the first is an exchange of sodium for potassium without alteration in water content of muscle. The second is an increase in cellular Na and water without a loss of K on a dry weight basis. In a series of studies in monkeys, Salmonella typhimurium sepsis was induced as an experimental model. Both patterns of muscle response to infection were detected. Electron probe microanalysis revealed that the loss of K concentration was due to an accumulation of intracellular saline which dilute the K content. The mechanism of this is unclear; however, a concomitant increase in undertermined osmoles in the serum suggests that there may be an increase in organic osmoles within the cell which leads to the dilution of intracellular K concentration.