Experiments were designed to test the hypothesis that ganglionic butyrylcholinesterase (BuChE) is derived from acetylcholinesterase (AChE). At 5 to 8 days following preganglionic denervation of the right superior cervical ganglion (SCG), cats were given sarin, 2.0 mumol/kg, i.v. At intervals of 1 h and 1, 2, 3, 6, 11, and 22 days later, they were killed, and the AChE and BuChE contents of both SCG and both stellate ganglia (StG) were assayed. The regeneration of AChE in the normal ganglia occurred in two phases: an initial rapid phase, to 25-40% of control activity in 1 day, and a slow phase, to approximately 70% of control activity in 22 days. BuChE reached approximately 85% of control activity in normal SCG and StG at 22 days. In the denervated SCG, AChE activity reached a maximum of approximately 17% of normal at 1 day, the value prior to the administration of sarin, and did not increase appreciably above this subsequently. BuChE activity in the denervated SCG reached approximately 50% of normal ganglia at 22 days. At each interval, its activity approached 55% of that of the contralateral normal SCG, the value found in the denervated SCG prior to the administration of sarin. Hence, the regeneration of BuChE appears to be independent of the presence of AChE in the neuropil. The origin of ganglionic BuChE remains obscure.