Uptake of horseradish peroxidase was studied by examining percentages of labelled synaptic vesicles in nerve endings of the excitatory axon innervating the opener muscle of the walking leg in the crayfish (Procambarus clarkii). Terminals on fibres with large excitatory postsynaptic potentials (EPSP) had higher percentage of labelled vesicles than terminals of fibres with small EJPs. The extent of labelling in the synaptic vesicle pool was greater for terminals with higher transmitter output. Evidence for three possible routes of synaptic vesicle formation was found. Movement of vesicles within the terminal as a whole appeared to be constrained, but rapid movement of vesicles within local populations probably occurs.