1. The effects on the release of transmitter by botulinum neurotoxins (BoNT; types A, B, E), tetanus toxin (TeTx), constituent chains or fragments were studied on identified cholinergic and non-cholinergic synapses in Aplysia. 2. Cholinergic synapses in the buccal ganglion were found to be greater than 100 fold more sensitive to extracellular application of BoNT than to TeTx whereas in non-cholinergic synapses of the cerebral ganglion the potencies of the toxins were reversed. When intracellularly applied TeTx and BoNT were found nearly equipotent. This disparity in the susceptibilities of BoNT and TeTx to inhibit transmission was attributed to differences in the toxin's acceptors or uptake systems in the two neurone types. 3. Micro-injection into cholinergic neurones of the isolated renatured toxins' chains showed that both light and heavy chains of BoNT are intracellularly required whereas the light chain of TeTx alone is sufficient. 4. The heavy chain of BoNT as well as that of TeTx were found to mediate internalization of active moieties via its amino-terminal half. Furthermore the heavy chain of one toxin could internalize the light chain of the other.