Summary Severely electroshocked dogs exhibit a striking fall in plasma volume, a sharp rise in plasma Na, blood sugar, muscle and plasma lactate but the hematocrit and hemoglobin show only moderate increases. The hypernatremia is fleeting in character and is largely, if not completely, independent of the decline in plasma volume; there is no evidence that water is shifted into cells causing shrinkage of the extracellular compartment. The increased concentration of the cation apparently results directly from electrostimulation of unknown neurogenic, Na regulating mechanisms in the central nervous system. The evanescent fall in plasma volume and simultaneous contraction of thiocyanate space in immediate post shocked dogs is not reflected by comparable changes in hematocrit and hemoglobin and is attributed to intense vasoconstriction due to sudden, violent shock to the brain and sympathetic nervous system with sequestration and trapping of large volumes of blood in the minute vessels at the vascular periphery. Closing off of substantial areas of the circulation by shock prevents free access of the dye T-1824 and SCN to the periphery of the vascular tree and thus accounts for the low post shock values of the plasma volume and SCN space compared to their controls.