This paper illustrates the principles of volume regulation in brain cells. Animal experiments were first performed ex vivo. Brains of gerbils were removed and incubated in 3 ml of physiological saline for 1 h. Control (0.86 g, n = 8) and swollen hemispheres (1.11 g, n = 8) were analyzed for tissue hydration, electrolytes and osmolality. The incubation media were also analyzed for gains or losses of electrolytes and water. Na+ and Cl- moved into and K+ moved out of the tissue. The ratio of Na+ influx to K+ efflux was calculated to be approximately 2:1. Water shifted into the tissue accompanying the net movements of small ions. In a simulated "cell" model constructed on the basis of the above observations with an outside saline and an inside colloid solution separated by a dialysis membrane, fluid shifts were demonstrated in the absence of (or even against) an osmotic gradient across the membrane under isobaric and isothermal conditions. Such paradoxical fluid shifts, presumably occurring in a similar manner to those in living cells, were shown to be due to the discharge of a huge thermodynamic potential accumulated by the cell as a condensation of ions outside and of proteins inside the cell membrane. We conclude that a loss in barrier function of the cell membrane ignites such a thermodynamic potential discharge causing an environmental fluid shift into the cells even under conditions of no (or even a contrary) osmotic gradient. Under such circumstances, countercotransporters and ion exchangers such as Na(+)-K(+)-2Cl- may work as modulators of the fluid shift, limiting its rate. The thermodynamic potential can explain the cascade of cell swelling (cytotoxic edema) as well as the spontaneous increase in osmolality in the ischemic cell when the cell volume increase is somehow restricted.