The activity of cytosolic creatine kinase in rat skeletal muscle rises stepwise during development. The increases occur simultnaeously with transient increases in DNA content. The second increase is accompanied by a rise in total protein, soluble sarcoplasmic protein and RNA/DNA ratio. Such changes are not observed at 20 days after birth, when creatine kinase finally accumulates to the adult level. Transient higher amounts of the MB and BB isoenzymes are observed after the first and second stepwise increase. The increase in creatine kinase activity observed after birth is predominantly due to an activation of the M gene. The BB isoenzyme is still present in adult skeletal muscle, but contributes little to the total activity.