The Indian red scorpion Buthus tamulus (or Mesobuthus tamulus) can cause fatal envenoming, but its mechanism of action is unclear. Venom was tested in vivo in anaesthetized rats and in vitro on isolated cardiac and skeletal muscle preparations. In vivo, the venom caused marked rhythmical fluctuations in blood pressure preceding cardiovascular collapse and death. On sheep Purkinje fibres, venom could induce spontaneous action potentials and cause prolongation of action potential duration. In chick biventer cervicis and mouse triangularis sterni preparations, venom enhanced the release of acetylcholine and induced repetitive firing of nerve action potentials in response to single shock stimulation. High concentrations caused stimulation then block of neuromuscular transmission. The main effects of Buthus tamulus venom are likely to be due to toxins that affect the opening of Na+ channels in nerves and muscles. This will cause an increase in the release of neurotransmitters in the peripheral nervous system, which may produce cardiovascular abnormalities and respiratory paralysis.