On superfusion of chromaffin granules from bovine adrenals with isotonic sodium and potassium salts, catecholamines and ATP were released in parallel and both in accordance with ion exchange kinetics. An artificial model was prepared by mixing a cationic (IRC-50) and an anionic (IR-4B) ion exchanger with COO- and NH+3 groups, respectively, as binding sites. This mixed ion exchanger showed in its storage and release of CA+ and ATP- striking similarities to the chromaffin granules. Within the pH range given for the interior of the granules--5.5-6--the artificial model even stored and released CA+ and ATP- within the same molar ratio as observed for the granules. We hypothesize that the chromaffin granule matrix in its storage and release functions operates as an amphoteric ion exchanger with COO- and NH+3 groups as the binding sites.