Altering [K+]o might have different effects on action potential duration (APD) in myocytes from different regions. Therefore, the effects of [K+]o on regional differences in action potential characteristics were investigated in sub-endocardial, mid-myocardial and sub-epicardial myocytes isolated from the base of guinea-pig left ventricular free wall using three different [K+]o (2.7, 5.4 and 8.1 mM KCl). Action potentials were recorded using the switch-clamp technique at 0.5 Hz. Increasing [K+]o from 2.7 to 8.1 mM shortened the action potential duration to 90 % repolarization (APD90; mean APD90 values in sub-endocardial, mid-myocardial and sub-epicardial myocytes were, respectively, 295 +- 9, 286 +- 9 and 266 +- 8 ms in 2.7 mM [K+]o, 270 +- 7, 255 +- 7 and 215 +- 7 ms in 5.4 mM [K+]o, 234 +- 7, 212 +- 10 and 155 +- 8 ms in 8.1 mM [K+]o), depolarized the resting potential, and reduced the amplitude of the action potential. The effect of increasing [K+]o on action potential characteristics was more pronounced in sub-epicardial myocytes than in sub-endocardial and mid-myocardial myocytes. The regional differences in APD90 in 5.4 mM [K+]o were increased in 8.1 mM [K+]o and abolished in 2.7 mM [K+]o. In conclusion, changing [K+]o produces more pronounced effects on action potentials in sub-epicardial myocytes than in sub-endocardial myocytes, modifying the normal heterogeneity of action potentials. The differences in the response of sub-epicardium and sub-endocardium to [K+]o may contribute to the flattening or inversion of the T wave commonly seen in patients presenting with hypokalaemia and the upright and tall T waves observed in electrocardiograms recorded during hyperkalaemia, although the underlying ionic currents remain to be determined.