There are M cells in the canine, rabbit, guinea pig, and human left ventricle (LV), but it is not known if they are present in the feline LV. Arterially perfused feline LV preparations were used for the recording of transmembrane action potentials from the epicardium (Epi), midmyocardium (M) and endomyocardium (Endo) under control conditions (n=12) and in the presence of I(Ks) blocker (chromanol 293B: 10 micromol/L, n=6) or I(Kr) blocker (E-4031: 2 micromol/L, n=6). The steady-state action potential duration at 90% repolarization and cycle length (APD90/CL) relation was obtained and fitted by the hyperbolic function APD(90) = CL/[(a x CL) + b]. In control, the shortest and longest action potential duration (APD) were observed in Epi and M, respectively, and the APD(90)/CL-relation curve was steeper in the M or Endo than in the Epi. Chromanol 293B prolonged APD in Epi, but not in M or Endo, resulting in no significant difference of the APD(90)/CL-relation curve among the 3 regions. E-4031 markedly, but homogeneously, prolonged APD in all regions, giving rise to decreased transmural dispersion of repolarization. In conclusion, there exists an M cell layer with a longer APD than the Epi and Endo layers and there is transmural electrical heterogeneity in the feline LV; however, the response to I(Kr) blocker is different from that of the canine LV probably because of species differences in the I(Kr) and I(Ks).