Liposomal encapsulation of the individual light and heavy chain of botulinum neurotoxin A was used to investigate their intra-cellular effects on synaptic transmission at the murine neuromuscular junction. Bath-application to phrenic nerve-hemidiaphragms of liposomes containing heavy chain (up to 75 nM) caused no alteration in neurally-evoked muscle tension. In contrast, liposomes with entrapped light chain (9-20 nM final concentration) gave a pre-synaptic blockade of neuromuscular transmission that could be relieved temporarily by 4-aminopyridine, as for the dichain toxin. Any contribution from contaminating intact toxin was excluded both by the purity and minimal toxicity in mice of the light chain preparations used, and by the lack of neuromuscular paralysis seen with liposomes containing the maximum amount of native toxin that could have been present in the light chain liposomes. As bath-application of high concentrations of light chain in the absence of liposomes failed to affect neurotransmiter release, it is concluded that this chain alone can mimic the action of the whole toxin inside mammalian motor nerve endings, its predominant site of action. Thus, light chain could provide a more effective probe for an intra-cellular component concerned with Ca2(+)-dependent secretion.