Persistent selective suppression of the butyrylcholinesterase (cholinesterase; acylcholine acyl-hydrolase, EC 3.1.1.8) activity of the superior cervical, stellate, and ciliary ganglia of cats by the daily administration of tetramonoisopropyl pyrophosphortetramide, 3.0 mumol/kg, intravenously, for 6 days produced a significant elevation in the levels of ganglionic acetylcholinesterase (acetylcholine hydrolase, EC 3.1.1.7). When the same procedure was preceded by the inactivation of over 95% of the ganglionic acetylcholinesterase by sarin, 2.0 mumol/kg, intravenously, the rate of regeneration of acetylcholinesterase was decreased. Results are interpreted as evidence that ganglionic butyrylcholinesterase may serve as a precursor to acetylcholinesterase, and that the level of butyrylcholinesterase may regulate the rate of acetylcholinesterase synthesis.