Most of our knowledge of storage and release mechanisms for catecholamines comes from work on the adrenal medulla (see N. & A. G. Kirshner, p. 279). There are important functional differences between the adrenal medullary cells and sympathetic neurons: in the latter the synthetic apparatus of the cell and the site of release are widely separated and re-uptake of released transmitter plays an important role in the maintenance of transmitter stores. At the ultrastructural level there is evidence for only a single population of catecholamine storage particles in each type of chromaffin cell in the adrenal medulla, whereas sympathetic neurons contain two kinds of catecholamine storage particles, whose distribution throughout the neuron is very different. It is not yet possible to explain the structural differences in terms of the functional differences. The finding that large dense-cored vesicles travel down the axon (see P. Banks & K. B. Helle, p. 305; B. G. Livett et al. , p. 359; A. Dahlstrom, p .325) suggests that they may be at least partly made in the cell body and play a role in supplying the terminal with some constituents synthesized in the cell body; whereas the finding that the small dense-cored vesicles are located almost exclusively in nerve terminals (Geffen & Ostberg 1969; Fillenz 1970) suggests that they are concerned with release, which presumably occurs mainly, if not exclusively, in this part of the neuron. In the present study Woods’s (1969) fixative has been used to re-examine the vesicles in the terminal axons of the smooth muscle coat of the normal rat vas deferens. The possible functional implications of the ultrastructural appearances are discussed.