The frequency of dark-cored vesicles is compared with the content in catecholamines in the adrenal medulla and carotid body of normal and reserpine-treated cats. Reserpine produces important diminution in the content of catecholamines but the frequency of the dark-cored vesicles does not parallel this descent. In the carotid body there is no relation between the content in catecholamines and the number of granules. The diameter of the dark cores does not change with the number of secretion vesicles present in normal or treated specimens. It is concluded that there is not partial discharge of the content of the vesicles. An all-or-nothing mechanism of discharge of exocytotic type is proposed.