K⁺ channels of isolated guinea-pig cardiomyocytes were studied using the patch-clamp technique. At transmembrane potentials between -120 and -220 mV we observed inward currents through an apparently novel channel. The novel channel was strongly rectifying, no outward currents could be recorded. Between -200 and -160 mV it had a slope conductance of 42.8 ± 3.0 pS (s.d.; n = 96). The open probability (P_o) showed a sigmoid voltage dependence and reached a maximum of 0.93 at -200 mV, half-maximal activation was approximately -150 mV. The voltage dependence of P_o was not affected by application of 50 µm isoproterenol. The open-time distribution could be described by a single exponential function, the mean open time ranged between 73.5 ms at -220 mV and 1.4 ms at -160 mV. At least two exponential components were required to fit the closed time distribution. Experiments with different external Na⁺, K⁺ and Cl^- concentrations suggested that the novel channel is K⁺ selective. Extracellular Ba²⁺ ions gave rise to a voltage-dependent reduction in P_o by inducing long closed states; Cs⁺ markedly reduced mean open time at -200 mV. In cell-attached recordings the novel channel frequently converted to a classical inward rectifier channel, and vice versa. This conversion was not voltage dependent. After excision of the patch, the novel channel always converted to a classical inward rectifier channel within 0-3 min. This conversion was not affected by intracellular Mg²⁺, phosphatidylinositol (4,5)-bisphosphate or spermine. Taken together, our findings suggest that the novel K⁺ channel represents a different 'mode' of the classical inward rectifier channel in which opening occurs only at very negative potentials.