Effects of D2O were studied on internodal cells of the freshwater alga Nitellopsis obtusa under plasmalemma perfusion (tonoplast-free cells) with voltage clamp, and on Ca2+ channels isolated from the alga and reconstituted in bilayer lipid membranes (BLM). External application of artificial pond water (APW) with D2O as the solvent to the perfused plasmalemma preparation led to an abrupt drop of membrane resistance (Rm = 0.12 +/- 0.03 k omega.cm2), thus preventing further voltage clamping. APW with 25% D2O caused a two-step reduction of Rm: first, down to 2.0 +/- 0.8 k omega.cm2, and then further to 200 omega.cm2, in 2 min. It was shown that in the first stage, Ca2+ channels are activated, and then, Ca2+ ions entering through them activate the Cl- channels. The Ca2+ channels are activated irreversibly. If 100 mM CsCl was substituted for 200 mM sucrose (introduced for iso-osmoticity), no effect of D2O on Rm was observed. Intracellular H2O/D2O substitution also did not change Rm. In experiments on single Ca2+ channels in BLM H2O/D2O substitution in a solution containing 100 mM KCl (trans side) produced no effect on channel activity, while in 10 mM KCl, at negative voltage, the open channel probability sharply increased. This effect was irreversible. The single channel conductance was not altered after the H2O/D2O substitution. The discussion of the possible mechanism of D2O action on Ca2+ and Cl- channels was based on an osmotic-like stress effect and the phenomenon of higher D-bond energy compared to the H-bond.