The amount of acetylcholine (ACh) and choline (Ch) in normal and denervated rat extensor digitorum longus (EDL) muscles, as well as that released spontaneously from these muscles, was determined by an extremely sensitive gas chromatographicmass spectrometric assay method. We found decreases in ACh content and spontaneous, resting ACh release as early as 8 h after denervation. The ACh content decreased to a plateau of 30% of control by 11 h; ACh release attained a plateau of 50% of control several hours later. These results showed that in denervated EDL muscles ACh content and spontaneous release (measured biochemically) decreased before nerve-evoked and spontaneous quantal release (measured physiologically) ceased at most synapses. The rapid reduction in ACh, or possibly in other substances that may be released with ACh, may be an important factor in initiating postsynaptic degenerative changes after nerve transection. Choline content and choline resting release increased significantly at both 1 and 3 days after nerve transection. These increases may be related to onset of postsynaptic neuromuscular degenerative changes.