Study objective — ATXII is a polypeptide toxin isolated from the sea anemone, Anemonia sulcata, known to delay sodium inactivation markedly and to induce early afterdepolarisations. The aim was to investigate the mechanism of its action. Design and materials — The mechanism of ATXII induced early afterdepolarisations was investigated in vitro in canine endocardial preparations using standard microelectrode techniques. Measurements and main results — ATXII (2 × 10−7 M) induced cycle length dependent prolongation of plateau, more marked in Purkinje than in muscle fibres, and early afterdepolarisations in Purkinje fibres only. The calcium channel antagonists verapamil (10−6 M, 10−5 M) and cobalt (2-4 mM), and drugs that block calcium release from the sarcoplasmic reticulum, ryanodine (10−6 M, 10−5 M) and caffeine (10 mM), did not antagonise the ATXII effects. However, tetrodotoxin (5 × 10−6 M) and lignocaine (4 × 10−5 M) shortened the action potential and suppressed early afterdepolarisations. The effects of lignocaine were seen at concentrations that did not significantly affect Vmax. Conclusions — ATXII induced early after depolarisations are due to the effects of ATXII on Na+ entry, probably via a slowly inactivated Na+ channel population. Calcium entry through the sarcolemmal Ca2+ channels and cyclic Ca2+ release from the sarcoplasmic reticulum are not required for the genesis of early afterdepolarisations in this model.