Substituted thiophenylalkenylcarboxylic acid guanidides, processes for their preparation, their use as a medicament or diagnostic, and a medicament containing them. There are described substituted thiophenylalkenylcarboxylic acid guanidides of the formula I ##STR1## in which R(1) to R(5) have the meanings given in the claims, which are outstandingly active antiarrhythmic pharmaceuticals having a cardioprotective component and are outstandingly suitable for infarct prophylaxis and infarct treatment and also for the treatment of angina pectoris, where they also inhibit or greatly reduce the pathophysiological processes in the formation of ischemically induced damage, in particular in the initiation of ischemically induced cardiac arrhythmias, in a preventive manner.