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A study has been made of the pharmacological actions of toxic preparations obtained from the box jellyfish Chironex fleckeri Southcott. Two toxin preparations were used. One was a tentacle extract which was partially purified by Sephadex gel filtration; the second was obtained by a process analogous to snake milking, and is probably similar in composition to the material injected into victims. All preparations were extremely toxic; death in animals, following minimally lethal doses, occurred in minutes. Respiratory arrest of central origin appeared to be the terminal event in all species tested. This was accompanied by marked signs of cardiotoxicity. The heart was slowed, irregular, and showed varying degrees of conduction delay. Terminally it showed atrioventricular block. Blood pressure changes were biphasic. An initial rise in carotid pressure was followed by a profound fall; a second rise to an above normal level frequently followed this. These blood pressure oscillations were damped down by prior treatment with hexamethonium but the hypertensive response remained. Blood samples taken before terminal apnoea showed a variable degree of haemolysis and a raised K+ level. Experiments with isolated organ preparations suggested that the toxin had a non‐specific lytic effect on cells, but did not contain pharmacologically active substances of small molecular weight such as 5‐hydroxytryptamine. It is suggested that the toxin(s) act by altering membrane permeability; the signs at death may reflect the sensitivity of the target organs to such a change.

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