1. Amino acids with a charged or a polar residue in the putative pore region, between lysine 346 and glutamate 372 of the alpha-subunit of the cGMP-gated channel from bovine rods were mutated to a different amino acid. The mRNA encoding for the wild-type, i.e. the alpha-subunit, or mutant channels was injected in Xenopus laevis oocytes. 2. When glutamate 363 was mutated to asparagine, serine or alanine, the current activated by a steady cGMP concentration declined in mutant channels. No current decline was observed when glutamate 363 was mutated to aspartate, glutamine or glycine, when theronine 359, 360 and 364 were mutated to alanine or when other charged residues in the pore region were neutralized. 3. The amount of current decline and its time course were significantly voltage dependent. In mutant E363A the current decline developed within about 1.5 s at -100 mV, but in about 6 s at +100 mV. In the same mutant, the current declined to about 55% of its initial level at +100 mV and to about 10% at -100 mV. 4. The current decline in mutants E363A, E363S and E363N was only moderately dependent on the cGMP concentration (from 10 to 1000 microM) and was not caused by a reduced affinity of the mutant channels for cGMP. Analysis of current fluctuations at a single-channel level indicated that current decline was primarily caused by a decrease of the open probability. 5. The wild-type channel was not permeable to dimethylammonium. When glutamate 363 was replaced by a smaller residue such as serine, mutant channels became permeable to dimethylammonium. 6. The current decline observed in mutant channels is reminiscent of desensitization of ligand-gated channels and of inactivation of voltage-gated channels. These results suggest also that gating and permeation through the cGMP-gated channel from bovine rods are intrinsically coupled and that glutamate 363 is part of the molecular structure controlling both the gating and the narrowest region of the pore.