The asymmetric (20S) form of acetylcholinesterase (AChE) in 1-day-old chick muscle is a hybrid enzyme containing both AChE (110 kd) and butyrylcholinesterase (BuChE, 72 kd) catalytic subunits. However, we now report that the asymmetric AChE extracted or immunopurified from older adult chicken muscles, where it is the endplate form, shows a progressive developmental loss of the BuChE subunit and its activities, centred around 4 weeks of age, while the AChE and collagenous subunits remain. In confirmation, using differential labelling and co-sedimentation it was shown that the hybrid 20S AChE/BuChE form of 1-day chick muscle is gradually and completely replaced during muscle maturation by a 21.3S form, also collagen-tailed but otherwise homogeneous in AChE catalytic subunits. Two other changes occur concomitantly. Firstly, the AChE catalytic subunit of the adult form has a lower apparent mol. wt in gel electrophoresis, by 5 kd, than the same subunit in the 1-day hybrid enzyme; this difference does not reside in the carbohydrate attachments. Secondly, the collagen tail changes, in that some conformation-dependent epitopes on it disappear in the same period. Hence, a major reorganization of the asymmetric AChE, involving all three types of subunit, occurs in the course of muscle development.