The effect of intradermally injected Phoneutria nigriventer venom (PNV) on vascular permeability of both rat and rabbit skin has been investigated. Oedema formation was measured as the local extravascular accumulation at skin sites of intravenously injected 125I-human serum albumin. In both rat and rabbit PNV induced dose-dependent oedema which was greatly potentiated by the vasodilators calcitonin-gene-related peptide and prostaglandin E1. In rats, PNV-induced oedema was markedly reduced either by previous treatment of the animals with the histamine H1 antagonist mepyramine and the serotonin antagonist methysergide or when venom was dialysed, indicating a major role for histamine and serotonin. In rabbits, dialysis of the venom to remove histamine and serotonin did not reduce PNV-induced oedema, indicating presence of oedematogenic component(s) which are different from the amines histamine or serotonin.