The distributions of acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) in the superior cervical ganglion (SCG) of the cat were determined by electron microscopy (EM) with the bis-(thioacetoxy)aurate (I), or Au(TA)2, method. Before the infusion of fixative, one of the enzymes was selectively, irreversibly inactivated in vivo, as confirmed by light microscope (LM) examination of sections of the stellate ganglion stained by the more specific copper thiocholine method. Physostigmine-treated controls, for inhibition of AChE or BuChE, were stained concomitantly with tissue for enzyme localization by the Au(TA)2 method for EM examination in each experiment. It was concluded that most of the AChE of the cat SCG is present in the plasma membranes of the preganglionic axons and their terminals, and in the dendritic and perikaryonal plasma membranes of the postsynaptic ganglion cells. BuChE is confined largely to the postsynaptic neuronal plasma membranes. Reasons for the discrepancies between the localizations found by the present direct EM observations and those deduced earlier from LM comparisons of normal and denervated SCG are discussed. It is proposed that a trophic factor released by the preganglionic terminals is probably required for the synthesis of postsynaptic neuronal AChE, and that BuChE may serve as a precursor of AChE at that site.