I K,slow1 and I K,slow2 are two important voltage-gated potassium (K+) currents expressed in mouse ventricular myocytes. However, their properties at the single-channel level have not been characterized. In this paper, we report two new single K+ channels, mK1 and mK2, in myocytes isolated from mouse ventricles and their possible correlation with the macroscopic currents I K,slow1 and I K,slow2. The conductance of mK1 and mK2 was 24 and 17 pS, respectively. Ensemble-averaged current demonstrated an inactivation time constant of 400 to 500 ms for mK1 compared with 1,300 to 2,000 ms for mK2. The mK1 channel was more sensitive than the MK2 channel to the K channel blocker 4-AP. In myocytes isolated from Kv1DN mice with functional knock out of the Kv1.5 channel, mK1 was not detectable but mK2 was present. Our data suggest that the newly characterized K+ channels, mK1 and mK2, likely correspond to the macroscopic currents of I K,slow1 and I K,slow2, respectively.