The quantitative and qualitative changes in fructose-P2 aldolase isoenzyme concentrations during development of "red" (leg) and "white" (breast) skeletal muscles of the chick were investigated. (a) The aldolase C to A subunit transition associated with muscle development is accompanied by large increases in aldolase activity (units/g, wet weight) and in specific catalytic activity (units/mg of protein). The accumulations in both muscle types follow pseudo-first order kinetics with doubling times of 2 to 3 days. The steady state level of aldolase activity in breast muscle (about 150 units/g) is approximately 4-fold higher than that in leg muscle (about 40 units/g). In contrast to leg muscle, the major increase in aldolase activity in breast muscle occurs during postembryonic development. (b) Immunotitration studies demonstrated a direct correlation between increases in enzyme activity and aldolase A subunits during postembryonic muscle development. It was calculated that under steady state conditions, aldolase A4 comprises about 1 percent and 0.26 percent, respectively, of the total wet weight of breast and leg muscle. (c) regulation at the level of protein synthesis in effecting the postembryonic accumulation of aldolase A4 in the muscle types was investigated in short term amino acid incorporation experiments. After a 1-hour pulse with [3H]leucine, aldolase from breast and leg muscle was isolated in a single step by affinity chromatography on phosphocellulose. Incorporation of tritum into aldolase A4 and into soluble or total protein was compared. Between 4 and 38 days after hatching, the rate of aldolase synthesis relative to the synthesis of soluble muscle protein increased about 7- and 3-fold, respectively, in breast and leg muscle. Relative to total protein, incorporation of [3H]leucine into A4 increased about 3-fold in breast muscle, and decreased slightly in leg muscle between 5 and 25 days after hatching. By 3 weeks after hatching, incorporation of [3H]leucine into aldolase A4 relative to incorporation into total protein was about 6-fold higher in breast muscle than it was in leg muscle. The present work, as well as other recent studies, are discussed in relation to the mechanism involved in controlling tissue-specific and stage-specific levels of aldolase isoenzymes in animal cells.