Internodal cells of a brackish water charophyte,Lamprothamnium succinctum (A. Br. in Ash.) R.D.W. regulate the turgor pressure in response to changes in both the cellular and the external osmotic pressures. During turgor regulation upon hypotonic treatment, net effluxes of K+ and Cl− from the vacuole, membrane depolarization, a transient increase in the electrical membrane conductance and a transient increase in concentration of cytoplasmic Ca2+ are induced. Activation of the plasmalemma Ca2+ channels and the Ca2+-controlled passive effluxes of K+ and Cl− through the plasmalemma ion channels are postulated.