A short-term toxicity study after 7 days oral daily administration of yessotoxin (YTX; 2 mg/kg/day), homoYTX (1 mg/kg/day), 45-hydroxy-homoYTX (1 mg/kg/day) and of the main diarrhoetic shellfish toxin okadaic acid (OA; 1 mg/kg/day) was carried out in mice. Symptoms, lethality, food consumption, body and organ weights, gross pathology and histopathology of the main organs and tissues, leukocytes formula as well as plasmatic levels of transaminases, lactate dehydrogenase and creatinine phosphokinase were evaluated. Heart tissue was studied also hystochemically for the presence of apoptotic nuclei and by transmission electron microscopy. No mortality, signs of toxicity or cumulative effects were induced by the repeated oral exposure to YTXs. Only ultrastructural changes in the cardiac muscle cells near the capillaries, such as package of rounded mitochondria and alteration of the cells boundary were observed, without any increase of lactate dehydrogenase, an index of cardiac damage. OA induced diarrhoea, body weight loss, reduced food consumption, and the death of 2/5 mice after 5 days. Necroscopy and/or light microscopy analysis revealed toxic effects mainly at forestomach (ulceration and hyperplasia), liver and, indirectly to body weight loss of mice, atrophic signs in the lymphoid organs and exocrine pancreas. Electron microscopy of heart tissue showed alterations of mitochondria and fibers in myocardiocytes, although no apoptotic change was recorded.