Abstract The effect of an extract from the venom gland of the black widow spider (BWSV) on transmission in the rabbit superior cervical ganglion was studied in vitro by electrophysiological techniques. Within 1 min after addition of BWSV to the bathing solution, spontaneous asynchronous postganglionic potentials were observed, but reduction of postganglionic action potentials evoked by stimulation of the preganglionic nerve occurred progressively over a 20–80 min period. BWSV had a selective depressive effect on one of the two major components of the compound postganglionic action potential; the second, S 1, spike being depressed more readily than the first, S 1, spike. Washing out the venom did not restore transmission but did stop the progressive reduction in postganglionic potentials. Increasing the calcium concentration three-fold in the external saline solution transiently stopped the progressive development of block. Repetitive stimulation of the preganglionic nerve at 5 or 10 Hz did not augment the BWSV-induced block.