1 Vesicular noradrenaline stores were compared in the heart, salivary gland and vas deferens of the rat. 2 Noradrenaline storage vesicles in nerve terminals of different organs differed with respect to the amount of noradrenaline they contain in the endogenous store (content), the amount of exogenous noradrenaline they can take up from the circulation (uptake) and the amount of noradrenaline they contain when they are saturated (total storage capacity). 3 The data suggest that the vesicles in the salivary gland and vas deferens are almost completely filled with transmitter while, in the heart, the vesicular store is filled to only 55% of its total capacity. 4 The monoamine oxidase inhibitor, tranylcypromine, was found to increase not only the size of the endogenous store but also the size of the unfilled store.