AbstractSome biological and neurochemical properties of the venom of stonefish (Syanceja horrida) were investigated. The venom exhibited oedema‐inducing, haemolytic, hyaluronidase, thrombin‐like, alkaline phosphomonoesterase, 5′ nucleotidase, acetylcholinesterase, phosphodiesterase, arginine esterase, and arginine amidase activities. Recalcification clotting time, prothrombin, and kaolin‐cephalin clotting times were increased 1.7–2.3‐ and 2.4‐fold respectively. The LD50(i.v. mouse) was 300 μg/Kg. Its effects on uptake and stimulation of neurotransmitter synthesis and release were observed in rat brain synaptosomes. In the presence of 100 μg venom, uptake of [methyl‐3H] choline in rat brain synaptosomes was inhibited 70%, while that of 4‐amino‐n‐[U‐14C] butyric acid was inhibited 20%. The toxin also stimulated the release of [3H]‐acetylcholine from the synaptoso