Observations on the fine structure of synaptic and non-synaptic axoplasm in the spinal cord of the lamprey ammocoete (Petromyzon marinus) are described. Previous studies on this material revealed a close association between spherical vesicles and axoplasmic microtubules near central synapses, and observations were consistent with the suggestion that synaptic vesicles become detached from oriented microtubules in the focal clusters adjoining the presynaptic membrane across which transmitter release takes place. These observations have been extended to include axons containing non-spherical or ellipsoidal vesicles-possibly containing a transmitter chemically and functionally distinct from that in spherical synaptic vesicles. Structural cross-bridges between both microtubules and vesicle populations are now described; these are found not only in the vesicle concentrations bordering synaptic foci, but also in non-synaptic axoplasmic regions where sparsely distributed vesicles are found in association with microtubules. It is suggested that the bridging between vesicles and microtubules may reflect a mechanism for transport of the former-a possibility in accord with cumulative evidence of involvement of microtubules in rapid intracellular translocation in a variety of cells. It is proposed that arrival of excitation at a synaptic site is not only coupled with transmitter release across the axon plasma membrane, but that events at the cell membrane may in turn be coupled with a means of supplying vesicles to the synaptic locus. This hypothesis suggests that the synapse may rely on distant parts of the neuron, perhaps including the cell body, for materials involved in synaptic transmission.