Abstract A system of two time‐dependent, first order differential equations of electrical membrane resistance and of hydrostatic pressure is derived. The equations are based on a set of flux equations for a 1,–1 valent electrolyte, combined with the equation of continuity, and the condition for conservation of mass. New insight into the functioning of the membrane oscillator is gained by the strict derivation of the relaxation equation of the averaged salt concentration with convection from the general partial differential equation. This relaxation equation is successfully transformed into the useful time‐dependent first order differential equation of the function of the electrical membrane resistance. Thereby the postulated relation for the concomitant changes of the electrical membrane resistance and the averaged salt concentration in the membrane phase is employed. The equations describe both the stationary and the non‐stationary states of Teorell's membrane oscillator. Particularly, the stationary pressure‐dependent, non‐monotonous current‐voltage curves and the current‐dependent, non‐monotonous pressure‐volume flux relations are calculated. These characteristics form the basis for mechano‐electrical transduction. Furthermore, the relaxation oscillations of membrane voltage, volume flux and hydrostatic pressure and the corresponding electroosmotic and hydrodynamic phase planes, representing stable limit cycles, are described. The stability criteria of the time‐dependent states are discussed.