1. The recapture and re-use of choline formed by the hydrolysis of released acetylcholine (ACh) was studied in the superior cervical ganglion of the cat using radioactive tracer techniques. The ganglion's ACh store was labelled by perfusion, during preganglionic nerve stimulation, with Krebs solution containing [(3)H]choline.2. Preganglionic stimulation (5 Hz for 20 min) of ganglia containing [(3)H]ACh released similar amounts of radioactivity when perfusion was with neostigmine-choline-Krebs or with hemicholinium-Krebs. This indicated that neostigmine does not increase transmitter release.3. The amount of radioactivity collected from stimulated ganglia during perfusion with choline-Krebs was 39% of the amount of radioactivity collected during perfusion with medium containing neostigmine or hemi-cholinium. This difference in release was almost (85%) accounted for at the end of the experiment by extra radioactive ACh in the ganglia perfused with choline-Krebs. It is concluded that during preganglionic nerve stimulation approximately 50-60% of endogenously produced choline is recaptured for ACh synthesis; thus, during activity preganglionic nerve terminals appear selectively to accumulate choline.4. However, chronically decentralized ganglia accumulated as much choline as did acutely decentralized ganglia, and this was interpreted as indicating that at rest preganglionic nerve terminals do not selectively accumulate choline.5. Increased exogenous choline concentration increased the amount of radioactivity collected during nerve stimulation in the absence, but not the presence, of an anticholinesterase agent. The spontaneous efflux of radioactivity was little affected by changes in external choline levels. It is concluded that exogenous choline and choline made available from released transmitter compete for uptake into nerve terminals.