The effect of Buthus martensi Karsch venom (MKV) on adrenergic responses was investigated using the rat isolated anococcygeus muscle (Acm), since several scorpion venoms can cause peripheral sympathetic nerve stimulation with enhanced adrenergic responses. The effects of phentolamine (5 microM), guanethidine (5 microM), tetrodotoxin (2 microM), desipramine (1.5 microM) and reserpine pretreatment in vivo (5 mg/kg s.c. x 24 hr and 5 mg/kg i.p. x 3 hr) on contractile responses of the rat Acm to field stimulation, noradrenaline (3 microM), tyramine (10 microM), crude MKV (2 micrograms/ml), carbachol (3 microM) and potassium chloride (50 mM) were compared. Phentolamine, guanethidine, tetrodotoxin and reserpine pretreatment completely blocked the contractile responses of the Acm to MKV and to field stimulation but desipramine potentiated the responses. The responses to NA were completely blocked by phentolamine, but were potentiated by guanethidine, desipramine and reserpine pretreatment. The contractile responses to tyramine were completely blocked by phentolamine, desipramine and reserpine pretreatment. The low doses (0.1 microgram/ml x 3) of MKV, which did not produce any observable increase in tone of the anococcygeus muscle, potentiated the contractile responses to field stimulation, but not the responses to exogenous NA. Thus, the adrenergic agonist action of MKV in the rat isolated anococcygeus muscle is mediated by some prejunctional mechanism(s) of action, presumably stimulating the release of the neurotransmitter noradrenaline.