1. A comparative study has been made of the effects of different fixatives on the ultrastructural appearance of granular vesicles accumulated against a constriction in cat hypogastric nerves. 2. After fixation in either osmium tetroxide or in glutaraldehyde followed by post-osmication accumulations of large granular vesicles (65-90 nm in diameter) were observed in profiles of swollen axons. 3. Fixation in acrolein and sodium dichromate revealed a second population of small granular vesicles (mostly 30-50 nm in diameter) in addition to the large vesicles seen after the other fixatives. 4. In reserpinized cats the small granular vesicles were absent. The large granular vesicles were much less numerous, irrespective of the fixative used. 5. It is suggested that either the small vesicles occur in normal noradrenergic axons but are only revealed by catecholamine-sensitive fixatives or they occur as a result of constricting the nerve trunk. Evidence for and against these possibilities is discussed.
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