Intracellular recording in outer segments of red rods of the toad retina showed that increasing Ca(2+) concentration in the perfusate mimicked certain aspects of light adaptation. Light sensitivity was reduced, the amplitude of light responses was reduced, the time course of light responses was altered by shortening the delay to the peak and increasing the decay rate, and the resting membrane potential was generally increased. These results provide further support for the hypothesis that Ca(2+) acts as an internal transmitter that is subject to light-induced release from the rod saccules. The addition of Ba(2+) to the perfusate mimicked dark adaptation by having effects opposite to all those described for Ca(2+). The Ba(2+) effects were specific, Mg(2+) and Sr(2+) being ineffective in similar or greater concentration. Also, Ba (2+) gave clear effects at concentrations as low as 0.1 mM. It is proposed that Ba(2+) enters the cell and reduces the uptake of Ca(2+) into the rod saccules, because the various effects of Ba(2+) upon light responses all seem explicable from this hypothesis. Ba(2+) also reduced the resting membrane potential, probably by reducing the membrane conductance for K(+). The demonstrated effects of Ba(2+) indicate an important role for Ba(2+) in analyzing the ionic aspects of transduction in vertebrate photoreceptors. They also suggest a critical physiological role for Ba(2+) in controlling both the resting properties and light-induced responses of vertebrate rods, because barium has been reported to be concentrated in outer and inner segments of cat photoreceptors.