Abstract Splenic nerve terminals contain noradrenergic vesicles which can be classified as small and large on the basis of both morphological and biochemical experiments. The origin of the large vesicles is likely to be the cell body and they are probably supplied to the nerve terminals by axonal transport. The origin of the small noradrenergic vesicles is still obscure so we have tested, using centrifugation techniques, the hypothesis that they are formed from the large noradrenergic vesicles during secretory activity in the nerve endings. Normally the small noradrenergic vesicles in resting splenic nerve terminals contain very little dopamine β-hydroxylase activity, as distinct from the large vesicles. However, after stimulation in vitro of the perfused spleen there is an appreciable amount of the enzyme associated with the small particles. These newly formed small vesicles are able to concentrate exogenous noradrenaline and their formation is dependent upon release of secretory products from the large vesicles as shown by their absence if the splenic nerve of a spleen perfused with a Ca 2+ -free solution is stimulated. A balance sheet has been drawn up which suggests that membrane-bound dopamine β-hydroxylase lost from the large noradrenergic vesicles upon stimulation is almost quantitatively recovered in the small noradrenergic vesicles. It is suggested that at least three types of noradrenaline storage particles exist in splenic nerve terminals (i) large vesicles which contain noradrenaline and dopamine β-hydroxylase and that can give rise to (ii) small vesicles which also contain both constituents and, (iii) vesicles which are virtually devoid of dopamine β-hydroxylase activity but which can store noradrenaline and have an origin quite different from those of the other two types.