1. Cats' adrenal glands were acutely denervated and perfused, in situ, with Locke's solution.2. When the medullary secretogogue Ba (2-10 mM) was added to the perfusion medium, large amounts of AMP and traces of ATP were found in the venous effluent from the adrenal gland along with the catecholamines. In this respect Ba mimicked the physiological secretogogue, acetylcholine.3. Control perfusions with known concentrations of ATP showed that under such conditions ATP was rapidly broken down within the adrenal vasculature.4. When such intravascular hydrolysis of ATP was suppressed by perfusing with a Ca-free, Mg-free Locke's solution containing 1-2 mM EDTA, catecholamine secretion induced by Ba was accompanied by the discharge of large amounts of ATP but relatively little AMP.5. The ATP that accompanied catecholamine secretion in such circumstances is assumed to derive from the ;heavy' nucleotide-rich chromaffin granules and it is concluded that the mechanism for catecholamine secretion does not depend on hydrolysis of the ATP within these granules.6. The release of ATP (unhydrolysed) supports the hypothesis that the chromaffin granules discharge their contents at the cell surface by the process of ;reverse pinocytosis'.