Cardioleputin, a new cardioactive toxin, was purified from a stonefish venom using column chromatographies. The purified toxin was found to be an unstable protein that was susceptible to heat and freeze-thawing. This protein showed to have a molecular size of 46,000 daltons, and its amino acid composition was rich in serine and glycine, but low in basic amino acids. The crude venom induced a sudden drop in blood pressure and heart rate of rats right after administration. Both the blood pressure and heart rate returned to their original values as time elapsed, and thereafter continued to show a gradual decrease. In addition, crude venom actively affected the contractile response and suppressed the heart rate of guinea pig atria. The purified toxin caused irreversible inotropical and chronotropical increases in guinea pig atria. The action of the toxin on the atria was completely different from that of lysolecithin. It might be suggested that the toxin acts on the Ca<img SRC="http:/img/fbpe/jvat/v2n2/image2184.gif"> ion channel of the atrial membrane.