Marine sponges are a rich source of novel biologically active substances such as haemolytic proteins, hemagglutinins and proteins with antimicrobial, antiviral and antifungal activity. Toxic water-soluble polymeric 3-alkylpyridinum salts (poly-APS) have been isolated from the marine sponge Reniera sarai. Poly-APS has pore-forming properties expressed in a dose-dependent manner. These interactions with cell membranes results in increased Ca2+ permeability. In in vivo experiments performed in rats, poly-APS caused transient bradycardia, prolongation of expirium, lowered blood pressure, formed numerous thrombocyte aggregates and caused death. The aim of the present study was to investigate the direct cardiotoxic effects of poly-APS on isolated rat hearts.