1. Native cGMP-gated channels were studied in rod outer segments of the larval tiger salamander, Ambystoma tigrinum. The alpha-subunit of the cGMP-gated channel, here referred to as the wild type (WT), and mutant channels were heterologously expressed in Xenopus laevis oocytes. These channels were studied in excised membrane patches in the inside-out configuration and were activated by the addition of 100 or 500 microM cGMP. The current carried by monovalent cations was measured under voltage-clamp conditions. 2. In the presence of 110 mM Na+ in the extracellular medium and different amounts of Na+ in the intracellular medium, the I-V relations of the native channel could be described by a single-site model with a profile of Gibbs free energy with two barriers and a well. A similar result was obtained in the presence of 110 mM Li+ in the extracellular medium and different amounts of Li+ in the intracellular medium. The well depth was 1.4RT (where R is the gas constant and T is the absolute temperature) for both Li+ and Na+. 3. The I-V relations of the native channel in the presence of 110 mM Na+ on one side of the membrane and 110 mM Li+ on the other side could not be described by the same single-site model with identical values of barriers and well obtained in the presence of Li+ or Na+ alone: the well for Li+ had to be at least 4RT. 4. In the presence of mixtures of 110 mM Li+ and Cs+ on the cytoplasmic side of the membrane, an anomalous mole fraction effect was observed both in the native and the WT channel. No anomalous behaviour was seen in the presence of Li(+)-Na+ and Li(+)-NH4+ mixtures. 5. The anomalous mole fraction effect with mixtures of Li+ and Cs+ was not observed in the channel where glutamate 363 was mutated to a glutamine (E363Q) or an asparagine (E363N). When glutamate 363 was mutated to an aspartate (E363D), the anomalous mole fraction effect with mixtures of Li+ and Cs+ was still observed, although significantly reduced. 6. When lysine 346, arginine 369, aspartate 370 and glutamate 372 were neutralized by mutation to glutamine, the ion permeation through the mutant channels and the WT channel had largely similar properties.(ABSTRACT TRUNCATED AT 400 WORDS)