The soldierfish (Gymnapistes marmoratus), which is related to the stonefish (Synanceia spp.), inhabits the western, southern and lower eastern coastlines of Australia. We have previously found that G. marmoratus venom possesses pharmacological activity similar to Synanceia trachynis venom (Hopkins, B.J., Hodgson, W.C., 1998. Cardiovascular studies on venom from the soldierfish (Gymnapistes marmoratus). Toxicon 36, 973-872; Church, J.E., Hodgson, W.C., 2000. Dose-dependent cardiovascular and neuromuscular effects of stonefish (Synanceja trachynis) venom. Toxicon 38, 391-407), namely an action at muscarinic receptors and adrenoceptors. The aim of this study was to determine the effectiveness of Synanceia antivenom in neutralising the in vitro and in vivo cardiovascular activity of G. marmoratus venom. Venom extract (0.4-12 microg protein/ml) caused dose- and endothelium-dependent relaxation in porcine U46619-precontracted coronary arteries. This relaxation was abolished by 10 min prior exposure of the tissue to Synanceia antivenom (3 units/ml). In rat paced (5 ms, 2 Hz, 7-12 V) left atria, G. marmoratus venom extract (40 microg protein/ml) produced a transient negative, followed by a sustained positive inotropic response. In spontaneously beating right atria, venom extract (40 microg protein/ml) produced similar changes in rate. Prior incubation of venom extract with Synanceia antivenom (1 unit/4 microg venom extract protein, 10 min) significantly attenuated both components of the inotropic response, and abolished the positive chronotropic response. In the anaesthetised rat, venom extract (400 microg protein/kg, i.v.) produced a transient depressor response, followed by a more sustained pressor response. Prior incubation of venom extract with Synanceia antivenom (1 unit/4 microg venom extract protein, 10 min) significantly attenuated both components of the in vivo response. As Synanceia antivenom neutralised the cardiovascular activity of G. marmoratus venom both in vitro and in vivo, we suggest that the venoms of the two species may share a similar component(s).