The brown-headed snake (Glyphodon tristis) inhabits the forest regions of Papua New Guinea, Torres Strait Islands, and far northern Queensland, Australia. Although bites by Glyphodon dunmalli have been reported, G. tristis was regarded as innocuous until 1989 when a healthy 20 year old man was bitten (Sutherland, S.K., Tibballs, J., 2001. Australian Animal Toxins, the Creatures, their Toxins and Care of the Poisoned Patient. University Press, Oxford). Treatment of envenomation by this species is empirical with no specific antivenom available. While no published studies on the venom of G. tristis are available, unpublished studies suggest neurotoxicity as being the main symptom of envenomation. In this study, the in vitro effects of G. tristis venom were examined using the chick biventer cervicis nerve muscle (CBCNM) preparation. Venom (10 microg/ml) inhibited indirect (0.2 ms, 0.1 Hz, supramaximal V) twitches of the CBCNM. This inhibition appeared to be presynaptic in origin as evidenced by the lack of effect of venom on responses to exogenous acetylcholine (1 mM), carbachol (20 microM) and KCl (40 mM) in the non-stimulated CBCNM. Prior addition (10 min) of polyvalent snake antivenom (5 U/ml; CSL Ltd) attenuated twitch inhibition. The venom (10-50 microg/ml) also appears to be myotoxic as indicated by a slowly developing contracture and inhibition of direct (2 ms, 0.1 Hz, supramaximal V, in the presence of tubocurarine 10 microM) twitches. Myotoxicity was confirmed by subsequent histological examination of tissues. This myotoxicity was prevented by the prior addition of polyvalent snake antivenom (30 U/ml). The phospholipase A inhibitor 4-BPB (1.8 mM) significantly attenuated the inhibition of indirect and direct twitches of the CBCNM preparation, indicating the involvement of a PLA2 component in the toxic action of the venom.