THe current suppressed by light, the generator current, was studied in solitary salamander (Ambystoma tigrinum) rod photoreceptors with the single-micropipette voltage-clamp technique. The effects of Ca, cyclic GMP, and voltage were measured while voltage- and Ca-activated currents of the inner segment were blocked with Co, Cs, and TEA (tetraethylammonium). The generator current was increased more than 5-fold by lowering the external Ca concentration from 1.5 mM to 10 microM. The generator current could be decreased approximately 1/2 by injecting Ca into an outer segment. Injection of EGTA quickly increased the generator current approximately 2-fold. After injection ceased, the increase was quickly reversed. The generator current could be increased more than 5-fold by injecting cyclic GMP or 8Br-cyclic GMP. Injection of protons, the pH buffer bicine (N,N-bis[2-hydroxyethyl]glycine), or GMP did not produce a change in the generator current. The current-voltage curve for the generator current was influenced by external Co: in 3 mM-Co the current-voltage curve had a negative resistance between -45 and -90 mV; in 0.1 mM-Co the current-voltage curve paralleled the voltage axis between -45 and -90 mV. The difference is attributed to a voltage-dependent block by Co. Susceptibility to the blocking action of Co was reduced by lowering internal or external Ca concentration, or by injecting cyclic GMP. When rods were bathed in a medium containing 7-100 microM-Ca, a step depolarization produced a time-dependent decline in current. Because the reversal potential remained constant, the decline is attributed to an inactivation. The extent of inactivation was reduced by increasing the concentration of external Ca or injecting cyclic GMP.