Adrenal medulla and carotid body of cats are studied with acrylic aldehyde-dichromate and glutaraldehyde-dichromate methods for catecholamines. The frequency of dark vesicles and the mean diameter of the cores obtained by these specific methods are compared with those found by the standard glutaraldehyde-osmíum method in normal and reserpine-treated animals. In the samples of normal organs fixed with aldehyde-dichromate methods, the number of dark-cored vesicles is significantly higher than in the controls. On the contrary, in the adrenal gland of reserpine-treated specimens the specific methods demonstrate reductions in the frequency of dark vesicles that can be semiquantitatively compared with the known diminution in the content of catecholamines produced by the drug. In the carotid body it was not found a so strict relation, nevertheless the specific methods give a consistently lower frequency of dark vesicles in reserpine-treated than in control animals. It is suggested that the discrepancy between standard and specific methods in the carotid body is due to substances existent in the dark cores contrasted by the former procedure but not by the latter. The constancy of the mean diameter of the cores even in glands that have lost 93 of their granules confirms the suggestion of an all-or-nothing exocytotic mechanism of discharge of the secretory vesicles. The inverse relation shown between the frequency of dark and clear vesicles indicates that membranes emerge in the cytoplasm after exocytosis as clear vesicles
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