The light-induced current as measured using a voltage clamp (holding voltage at resting potential) is attenuated when sodium ions in the bathing solution, Na(o), are replaced by Tris, choline, or Li or when NaCl is replaced by sucrose. After replacement of NaCl by sucrose, the reversal voltage, V(rev), for the light response becomes more negative. In this case, the slope of the V(rev) vs. log Na(o) near Na(o) = 425 mM is approximately 55 mV/decade increase of Na(o) (mean for 13 cells). The slope decreases at lower values of Na(o). Choline is not impermeant and partially substitutes for Na; the slope of V(rev) vs. log Na(o) is 20 mV/decade (mean for three cells). V(rev) does not change when Na is replaced by Li. Decreases in the bath concentrations of Ca, Mg, Cl, or K do not affect V(rev). When Na(o) = 212 mM, V(rev) becomes more positive when K(o) is increased. Thus, light induces a change in membrane permeability to Na and probably also to K.