Creatine kinase (CK) and acetylcholinesterase (AChE) were followed during the course of development of embryonic muscle in tissue culture for periods of up to 8 weeks. Control muscle cultures released CK and AChE into the medium for the first 2-3 weeks, after which time measurable enzyme release ended. In dystrophic muscle cultures release of CK and AChE continued over most of the 8-week culture period. Cumulative measurable activity of CK was significantly greater than that of controls by the 3rd week and thereafter and cumulative AChE release was greater than that of controls by the 5th week and averaged about twice that of controls by the end of the culture period. Total cell CK activity of the dystrophic muscle cultures was greater than control values but results of cellulose acetate electrophoretic analysis of CK isozyme composition indicated that control muscle cultures attained higher percentage levels of MM isozyme type and lower levels of MB isozyme. Breast muscle extracts from adult dystrophic chickens had an approximately 10--13% content of MB isozyme while in similar extracts of control chicken muscle only the MM isozyme was detectable and total activity was greater than in dystrophics. Bound AChE activity of dystrophic muscle was significantly greater than control levels subsequent to the 2nd week in culture. Soluble AChE activity of dystrophic muscle was somewhat greater than comparable control activity in 2 of 3 tissue culture series at approximately the same period. In 7--10 week-old dystrophic chickens, both soluble and bound AChE activities of dystrophic breast muscle extracts were markedly increased over control values.