The magnitude of the cytosolic pool of acetylcholine in the cholinergic electromotor nerve terminals of Torpedo marmorata has been calculated to be 22 +/- 3% of the total by comparing the isotopic ratio of acetylcholine with that of choline when slices of electric organ were incubated with 10 microM deuterated choline. The calculation is based on a two-compartment model that assumes the presence, in unstimulated tissue, of a vesicular pool of acetylcholine that does not exchange, under resting conditions, with a second cytosolic pool; the latter, by contrast, is subject to 'futile recycling' and comes into isotopic equilibrium with the tissue choline pool.