The ability of the two-chain form of tetanus toxin (TeTx), its constituent light (LC) or heavy (HC) chains, and papain fragment to block evoked acetylcholine (ACh) release in the buccal ganglia of Aplysia californica was studied electrophysiologically. Extracellularly applied, TeTx or its B fragment (consisting of LC and beta 2, the amino-terminal portion of HC) blocked ACh release, whereas LC, HC, or the beta 2 fragment did not affect it. Toxicity was restored when LC was bath applied together with HC or the beta 2 fragment. When injected into the presynaptic neuron, TeTx, the B fragment or LC, but not HC, induced inhibition of ACh release. These results indicate that the blockade of ACh release by TeTx is mimicked by intracellular action of LC, the internalization of which is mediated by the HC via its amino-terminal moiety.