Exposure of Limulus abdominal ganglia to (+)-tubocurarine before or after cotton-mouth moccasin venom treatment resulted in a decrease of spontaneous activity in individual fibers in dorsal nerve roots and an inhibition of heart rate, but did not decrease responsiveness of the dorsal nerve to tactile stimulation. No effects of curare were obtained in nonsynaptic regions of nerve bundles either with or without venom pretreatment. Combining acetylcholine perfusion with (+)-tubocurarine resulted in a suppression of the stimulating effect of acetylcholine on dorsal nerve activity.