The cGMP-dependent channel protein has been purified from bovine rod photoreceptor membranes and incorporated into planar lipid membranes. At low divalent cation concentrations, cGMP stimulated single-channel current fluctuations. The probability Po of the channel being open strongly depended on the cGMP concentration (EC50 = 31 microM; Hill coefficient, n = 2.3); whereas the single-channel conductance (lambda = 26 pS) was independent of the agonist concentration. The agonist-stimulated increase in the probability of an open channel was largely due to shorter closed times and, to a lesser extent, due to the channel staying open for a longer time. The current-voltage relationship of the single open channel deviated from ohmic behavior, and the open probability decreased at more negative membrane potentials. The rectification of the macroscopic cGMP-induced current in artificial bilayers that contained many channel copies can be accounted for by the voltage dependence of channel gating together with the nonlinearity of the current-voltage curve of an open channel. Current fluctuations exhibited a variety of sublevels, indicating that the channel may exist in more than one conductive state.