The properties of the cyclic-GMP-activated conductance in the plasma membrane of bovine rod outer segments were studied in excised membranes. Multiple-channel and single-channel currents were recorded by the patch-clamp technique in symmetrical NaCl solutions which were free of divalent cations. The current-voltage relationship for the current, recorded when a large population of channels was activated, exhibited outward rectification. Rectification decreased as the concentration of cyclic-GMP was increased, and the concentration of cyclic-GMP required for half maximal activation of the channel decreased with depolarization. At a concentration of 1-3 microM cyclic-GMP, single-channel activity could be observed from these excised patches. The conductance of the open channel was 6 pS and was independent of the membrane potential. These results are consistent with the interpretation that under these conditions, the mechanism responsible for the outward rectification is due to an increase in the probability of an open channel as the membrane is depolarized. The cyclic-GMP-activated current could be blocked by L-cis-diltiazem. Block was voltage and time dependent. The time constant for the onset of block and its steady state level increased with depolarization. The extent of block by diltiazem was not enhanced as the cyclic-GMP concentration was increased, suggesting that the channel is not required to be open for block to occur. Complete block was never attained even for high concentrations of diltiazem. However, the diltiazem-resistant component of the cyclic-GMP-activated current could be blocked by tetracaine.