1 The effects on mammalian autonomic neuromuscular transmission of the crude venom of the female Sydney funnel-web spider Atrax robustus, have been investigated.2 At doses of 10 mug/ml or lower the indirectly elicited twitch-like responses of the rat anococcygeus preparation were inhibited. At doses greater than 10 mug/ml there was an initial reduction in the twitch-like response followed by a sustained contracture of the tissue.3 The long-lasting contracture caused by the venom was abolished by the application of phentolamine. It was virtually non-existent in muscle preparations isolated from reserpine-treated rats.4 In the presence of tetrodotoxin the contracture was smaller and less well maintained than in its absence.5 The venom caused a small reduction in the amplitude of the indirectly elicited twitch-like response of the longitudinal muscle of the guinea-pig ileum, followed by an increase in the tone of the preparation. The increase in tone was maintained for several minutes and was rapidly abolished by the application of atropine. The presence of venom did not affect control responses to either histamine or acetylcholine.6 Inhibitory transmission in the rat anococcygeus preparation was unaffected by the venom.7 The neurally-mediated twitch-like responses of both guinea-pig and rat vas deferens were inhibited by the venom at doses below 10 mug/ml. At higher doses the inhibition was accompanied by spontaneous contractions, and at doses in excess of 100 mug/ml the inhibition of twitch-like responses was transient and was followed by a potentiation of the motor response and extensive spontaneous activity. The preparation became quiescent 20 min after the application of venom and the evoked response was abolished after 60 min.8 The venom had qualitatively similar effects on motor transmission in the human vas deferens as on the rat and guinea-pig preparations. However, the human preparations were 50 to 100 times more sensitive to the effects of the venom.