Fish venoms can be lethal for Vertebrates. The effect depends of dose and subject, more than incriminated fish. The most constant symptom is a violent pain; but the serious pharmacological effects are respiratory and heart failure with marked hypotension and cardiac perturbations, neurologic damage, such as seizure and coma. Experimentation is difficult due to venom instability. Activity is lost by distilled water, lyophilisation in buffers, several successive freezing and defreezing. In addition, when venom is broken, other pharmacological effects are evidenced, for instance, with Synanceia verrucosa venom, hypertensive phase takes the place of hypotension. It is difficult to distinguish toxin effect from this of denaturation products of the toxin. Noradrenaline is present in Synanceia venom, and it seems that acetylcholine exists in some venom, at least when diluted in saline solution. Other biological active products are present. Purified toxins allow pharmacological investigations. Stonefish venom is better studied, because venomous glands contain relatively high venom quantity. Stonustoxin from Synanceia horrida exerts its action through NO-synthase liberation, and its primary action can be attributed to its potent vasorelaxant activity, causing a rapid, marked and irreversible hypotension. Trachynilysin, from Synanceia trachynis, causes massive release and depletion of acetylcholine and damage to nerve and muscle fibres, which can account for the inhibition of neuromuscular function, and skeletal paralysis. But the used doses are not compatible with respiratory arrest. Verrucotoxin from Synanceia verrucosa activates potassium channels dependent from ATP; this can explain damage, and probably neurologic and respiratory distress.