This study aimed to define the complete concentration-effect relationship for anticurare effects of 3, 4-diaminopyridine (3, 4-DAP) in the isolated sympathetic ganglion of the bullfrog. Synaptic transmission was monitored by extracellular and intracellular recordings of the postganglionic response to preganglionic stimulation. A previous study showed that in the bullfrog sympathetic ganglion 3, 4-DAP caused stimulus-bound repetitive postganglionic responses (SBR) to each single preganglionic stimulus. The concentration-effect relationship for 3, 4-DAP-induced SBR was bell-shaped, and the descending limb of the curve reflected progressive suppression of SBR while normal synaptic transmission was maintained. In the present study a detailed concentration-effect analysis of 3, 4-DAP's anticurare action also resulted in a bell-shaped curve nearly congruent with that for SBR. SBR and anticurare effects of 3, 4-DAP therefore occupy a common concentration-effect domain, and this suggests that a common mechanism (increased transmitter release) may account for both effects.