The mechanism underlying the Ba2+-induced automaticity was studied in isolated guinea pig ventricular myocytes using the whole-cell clamp method and a patch electrode. In the presence of 0.1-0.3 mM Ba2+, application of a weak depolarizing current induced repetitive firing of spontaneous action potentials. Application of tetrodotoxin or Ca2+ channel blockers and removal of external Na+ and Ca2+ did not abolish the rhythmic activity, thereby suggesting that activation of inward currents played no crucial role in the generation of this rhythmicity. Voltage-clamp studies revealed that Ba2+ blocked the inward rectifier K+ current (iK1) in a voltage- and time-dependent manner with a greater and more rapid block at more negative potentials. This Ba2+ action could quantitatively be fitted with a model of the conventional bimolecular adsorption isotherm with a voltage-dependent dissociation constant. Simulation studies using this model showed that a membrane model, in which only the iK1 system and a small leak conductance were incorporated, could reproduce an automatic activity similar to that seen experimentally. Thus, in isolated ventricular cells, the voltage- and time-dependent block of iK1 by Ba2+ appears to be one important mechanism underlying the automaticity.