Hadruroides lunatus venom has a low lethality in mice (ld50, 68 mg/kg i.p.) and in dogs induces a fall in blood pressure which is prevented by an antihistaminic. The venom liberates acetylcholine and potentiates acetylcholine and serotonin actions in the rat ileum. It does not affect rat phrenic nerve-diaphragm nor toad rectus abdominis isolated preparations, but increases basal tone of the isolated atrium of the rat. © 1981.