Abstract Transmembrane potential changes were monitored in 21 dogs that were shocked by intravenous injection of Difco purified endotoxin (055:B5). Corresponding serial measurements of electrolyte concentration in plasma and muscle biopsies were obtained to assess fluid and electrolyte changes. During shock the transmembrane potential was found to become significantly less negative (−55.2 mv.) from a control of −87.5 mv. (p < 0.001). A significant efflux of K + (p < 0.02) from the cell was recorded, but intracellular Na + and Cl − concentration rose. A plausible explanation for the fluid and electrolyte shifts, possibly due to a decrease in the muscle temperature and a resultant decline in metabolism, has been offered.