An acidic phospholipase A2 (OHV A-PLA2) purified from Ophiophagus hannah venom had a cardiotoxic action on rat heart. In rats OHV A-PLA2 (2-4 mg/kg) caused ECG abnormalities including decreased heart rate, prolonged P-R interval, widened QRS complex and complete A-V block. When tested on isolated rat right atria, OHV A-PLA2 (10-20 micrograms/ml) produced a positive chronotropic effect. When tested on isolated rat left atria or papillary muscle preparations, OHV A-PLA2 (2.5-20 micrograms/ml) caused positive inotropic effect, followed by contracture. The positive inotropic effects could be abolished by high Ca2+ and enhanced by low Ca2+; both treatments accelerated contracture. The contracture could be inhibited in Mn2+ (5 mM)-containing medium and abolished by Ca(2+)-free bath solution containing 1 mM EDTA. The cardiotoxic action of OHV A-PLA2 was not influenced by verapamil, tetrodotoxin, propranolol, phentolamine, atropine or indomethacin. It is suggested that the cardiotoxic effects of OHV A-PLA2 may result from increasing intracellular levels of Ca2+.