developmental regulation, from the fetal period to 11 months of age, and the influence of denervation on the appearance and disappearance of the molecular forms of acetylcholinesterase (ache) and butyrylcholinesterase (buche) in rat skeletal muscle were examined three principal forms, denoted by sedimentation coefficients of 4, 10.8, and 16 s, were observed in muscle from all age groups. the amounts of each of the molecular forms of ache and buche in skeletal muscle exhibited distinct and reciprocal patterns of appearance and disappearance during pre- and postnatal development. in tissue derived from animals less than 2 weeks of age, buche represented the predominant component of activity in the 4 s form, was present equally with ache in the 10.8 and was subordinate to ache in the 16 s form. denervation of muscle caused a dramatic reduction in the reduction in the presence of ache molecular forms with no discernable influence on the presence of buche molecular forms. these results indicate that biosynthesis of buche is strictly regulated in a reciprocal synthesis of ache, and that buche metabolism is independent of the state of muscle innervation. increased synthesis of ache and either reduced synthesis or increased degradation of buche can account for the reciprocal regulation of these enzymes