Acetylcholinesterase and butyrylcholinesterase activities were demonstrated in transverse sections of the rabbit ear artery using a "direct coloring" thiocholine technique. BW 284C51, iso-OMPA and DFP were used as inhibitors, and acetyl and butyrylthiocholine iodides as substrates. Both enzymes caused stain deposits at the medial-adventitial border of the artery, in a region where sympathetic nerves are present. Sections treated to demonstrate acetylcholinesterase showed staining which was lighter than that for butyrylcholinesterase. Degeneration of the sympathetic nerves following superior cervical ganglionectomy virtually abolished stain at the medial-adventitial border. These results provide some support for the evidence of other workers that a cholinergic mechanism may be linked to sympathetic nerve transmission in the rabbit ear artery.