Demonstration of choline acetyltransferase activity in the caotid body of the cat.

1. The distribution of choline acetyltransferase in the carotid body of the cat has been investigated with the electron microscope to determine sites of enzymic activity. This is of relevance to the possible role of acetylcholine as a transmitter in the carotid body.2. Tissues were fixed for short periods and incubated by the method of Kasa, Mann & Hebb, for the fine structural localization of choline acetyltransferase.3. The enzyme was found in the cytoplasm of the type I cells and seemed to be associated with vesicles. No enzyme was found in the large nerve endings synapsing with the type I cell.4. Whole carotid bodies were assayed for their choline acetyltransferase activity and significant amounts were found.5. It is concluded that acetylcholine may be a transmitter in the carotid body and that it is synthesized in type I cells. A possible mode of initiation of chemoreceptor afferent impulses is suggested.