The fine structural localization of cholinesterases in the carotid body of the cat.

1. The distribution of cholinesterases in the carotid body of the cat has been investigated with the electron microscope to obtain a clearer picture of the localization of the enzyme. 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 Karnovsky for the fine localization of cholinesterases.3. The enzyme was found in two main sites. Butyrylcholinesterase was present on the type II cell membrane while acetylcholinesterase was found in groups of peripheral axons. No intracellular enzyme was found.4. It is concluded that acetylcholine may be a transmitter in the carotid body and that it may be partly destroyed by enzymic hydrolysis. A possible explanation for some of the anomalous pharmacological findings is suggested.