Electrophysiological effects of intracoronarily administered contrast media have been documented in 12 thoracotomized dogs at the cellular level by use of a modified microelectrode technique. Injections (n = 63) of 4 different contrast media uniformly led to a temporary cellular hyperpolarisation of the resting potential and prolongation of the action potential. Additional experiments with intracoronary injections of several electrolyte solutions indicate that the observed effects can be explained by contrast-induced changes in the extracellular electrolyte concentrations, mainly by a local deficiency of potassium ions and an excess of sodium ions. The significance of the findings for mechanisms underlying ECG-changes and ventricular arrhythmia by radiographic contrast media will be discussed.