The source of butyryicholinesterase (acylcholine acylhydrolase, EC 3.1.1.8) in the ganglion cells of the cat superior cervical and ciliary ganglia has been elusive, inasmuch as the enzyme is present in high concentrations in the neuropil, where it is confined largely to the dendritic and perikaryonal plasma mem-branes, but appears to be absent from the perikarya. In the present study, ganglionic butyrylcholinesterase was near-totally inactivated by the injection of tetramonoisopropyl pyrophosphoramide (6.0 jmol/kg of body weight) intravenously. During the ensuing 72 hr, the regenerating enzyme became detectable by the copper thiocholine histochemical method in the somata of essentially all ganglion cells and in the neuropil. Results were similar in preganglionically denervated superior cervical ganglia and in normal ciliary ganglia. These findings suggest (i) that butyrylcholinesterase indeed is synthesized in the ganglion cell perikarya (presumably, the rough endoplasmic reticulum) and transported extremely rapidly to more peripheral cellular sites and (ii) that the synthesis is largely independent of control by any neurotrophic factor provided by the preganglionic axonal terminals. Similar studies were conducted in the rat. In this species, in contrast to the cat, the somata of essentially all ganglion cells of the superior cervical ganglion contain various but at least moderate concentrations of acetylcholinesterase (acetylcholine acetylhydrolase, EC 3.1.1.7) and propionylcholinesterase (acylcholine acylhydrolase, EC 3.1.1.8). After injection of tetramonoisopropyl pyrophosphoramide, propionylcholinesterase reappeared in the ganglion cell somata before its accumulation in the neuropil, as would be expected.There is a marked species difference between the cat and rat with regard to the distributions of acetylcholinesterase (AcChoEase; acetylcholine acetylhydrolase, EC 3.1.1.7) and nonspecific cholinesterase (acylcholine acylhydrolase, EC 3.1.1.8) in the superior cervical ganglion (SCG) and other autonomic ganglia.In the cat, light microscopic examination by the highly specific copper thiocholine (CuSCho) method has shown that AcChoEase is present in the perikarya of all ganglion cells of the SCG (1); its concentration there is high in a small proportion (<1%; refs. 1 and 2) and moderate or low in the remainder; a high concentration of AcChoEase is also present in the neuropil. The latter is comprised of preganglionic axons and their terminals, ganglion cell dendrites, and Schwann sheath cells; these elements are indistinguishable by light microscopy. However, more recent electron microscopic (EM) examination by the bisthioacetoxy aurate method (3) has clarified the cytological location of AcChoEase in the neuropil (4, 5). The nonspecific cholinesterase (or butyrylcholinesterase; BtChoEase) of the cat SCG appears by light microscopy to be confined to the neuropil and absent from the ganglion cell perikarya (6). It was concluded on this basis and from other studies (7, 8) that it is located in the Schwan...