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The effects of peroxynitrite (ONOO-) on cultured cardiac myocytes were examined by simultaneous measurements of intracellular Ca2+ (Ca2+]i) and contractile function. On exposure to 0.2 mM ONOO-, Ca2+]i increased to beyond the systolic level within 5 min with a concomitant decrease in spontaneous contraction of myocytes followed by complete arrest. Addition of a L-type Ca2+ channel blocker or removal of extracellular Ca2+ prevented the ONOO(-)-induced increase in Ca2+]i, indicating that the increase in Ca2+]i was caused by the enhanced influx of Ca2+ through the plasma membrane and not by the enhanced release from sarcoplasmic reticulum (SR). Plasma membrane fluidity and concentration of the thiobarbiturate acid-reactive substance (TBARS) in the cells remained unchanged by the ONOO- treatment. The complete cessation of contraction of myocytes persisted even under the massive increase in Ca2+]i, which was induced by an additional saponin (5 microM) treatment. In conclusion, ONOO- increases Ca2+]i in myocytes through disturbance of Ca2+ transport systems in the plasma membrane and impairs contractile protein.