During an ultra-structural study of the inferior olive and of other nuclei in the central nervous system of the cat it was observed that in formalin and glutaraldehyde perfused material many normal boutons in these regions contained sy nap tic vesicles of a special type. The vesicles were elongated instead of round, and are usually 170-220X500 A in size. Boutons containing elongated vesicles may have only vesicles of this type, or they may have a mixture of elongated and ordinary synaptic vesicles. Elongated vesicles apparently occur in the boutons when aldehydes are used as fixatives. In aldehyde fixed material from the ventral lamella of the interior olive about 40% of the normal boutons have such vesicles. The phenomenon is probably not related to tonicity changes in the tissue.