1. The effect of whole tetanus toxin (TeTX) and of its light chain (TeTX L-chain) on transmitter release was determined by presynaptic pressure-injection in the squid giant synapse. 2. The results indicate that whole TeTX does not modify transmission while the L-chain blocks transmission within 20-30 min. This block does not involve changes in the sodium or potassium conductances responsible for spike generation or the voltage-dependent presynaptic calcium current responsible for transmitter release. 3. Western blotting of protein fractions from the squid optic lobe demonstrated the presence of a protein which reacted with specific antibodies against mammalian synaptobrevin, a vesicular protein. In addition, this protein was enzymatically cleaved by the L-chain component of the toxin in a similar fashion to its mammalian counterpart. 4. These results demonstrate that TeTX L-chain toxin acts directly on a squid synaptobrevin and prevents synaptic release probably by interfering with the docking-fusion synaptic vesicles at the active zone.