We have investigated the mechanisms underlying the cardiac effects of the toxin from the box jellyfish Chironex fleckeri. Papillary muscles isolated from the hearts of ferrets and ventricular myocytes isolated from the hearts of ferrets and rats were used. Force, intracellular [Ca2+], and membrane potential were monitored in the papillary muscles; contraction, intracellular [Ca2+], intracellular [Na+], and membrane currents were monitored in the isolated myocytes. Application of the toxin to these preparations resulted in a large increase in intracellular [Ca2+] and the adverse symptoms of Ca2+ overload (aftercontractions, spontaneous contractions, a decrease in developed force, and an increase in resting force). The response of papillary muscles to the toxin was not inhibited by blockers of Ca2+ or Na+ channels or by inhibitors of the sarcoplasmic reticulum, Na+/K+ ATPase, or Na+/H+ exchange. The response to the toxin was, however, blocked by prior exposure to a solution which contained no Na+ and by Ni2+. In the isolated myocytes, as well as an increase in intracellular [Ca2+], the toxin also caused an increase in intracellular [Na+] and the appearance of a current which was inward at negative potentials and reversed at about -10 mV. These data can be explained by the toxin increasing Na+ influx into the cell. The increase in intracellular [Na+] will then increase intracellular [Ca2+] via the Na+/Ca2+ exchange mechanism, thus producing the observed Ca2+ overload.