Previous studies have suggested that trophic action of nerves after hatching suppresses embryonic properties of chick muscle acetylcholinesterase (AChE). Normal chick muscles have low AChE activity which is localized at myoneural junctions and consists of a single major high molecular weight isozyme form. Denervation results in an increase of extrajunctional AChE activity and return of two low molecular weight AChE isozymes characteristic of embryonic chick muscle. The increase in AChE activity following denervation was reversibly depressed by direct electrical stimulation of muscle sufficient to elicit contraction, while nonspecific cholinesterase (BChE) and creatine phosphokinase (CPK) activities were not affected by stimulation. No effect on the increase of AChE activity was obtained by denervating muscles leaving long or short nerve stump lengths. The results suggest that contractile activity may be involved in trophic regulation of chick muscle AChE properties.