The cardiotoxicity of high concentrations of inhaled difluorodichloromethane (FC 12) has now been acknowledged. In the present study, the effects of FC 12 on the electrical activity of cells in the atrial and ventricular myocardium of anesthetized rats were recorded with "flexibly mounted" intracellular microelectrodes. The major phenomena observed in both types of cells were: a distinct decrease in the diastolic potential, a decrease inthe amplitude of the action potential, modifications in the shape of the action potential. Analysis of the simultaneously obtained electrocardiogram specifies the rhythm abnormalities which consist of an important decrease in the atrio-ventricular conduction and changes in the myocardial excitability. The cardiotoxicity of FC 12 is assumed to affect passive or active transmembrane ionic movements. Possible mechanisms are suggested here.