Exposure of the rabbit lens, in vitro, to 10(-3) M-hydrogen peroxide resulted in an increase of the lens sodium, calcium and water content together with a decrease of the lens potassium content. No such changes were observed in the presence of 10(-4) or 10(-5) M-hydrogen peroxide. 10(-3) M-hydrogen peroxide was observed to diminish 86Rb uptake by 32% and to reduce the lens Na+-, K+-ATPase activity by 37%. The lens potential remained ouabain-sensitive after peroxide treatment. These findings suggest that only partial inhibition of the lens sodium pump takes place in the presence of 10(-3) M-hydrogen peroxide. Lens depolarization, increased lens conductance and an increase rate of 86Rb efflux were also observed upon exposure of the lens to 10(-3) M-hydrogen peroxide. Such alterations of 86Rb efflux and electrophysiological parameters indicate that hydrogen peroxide changes the passive permeability of lens membranes.