1. The isometric properties and related biochemical properties of a developing rat fast muscle (extensor digitorum longus, EDL) have been determined during the 21 days after birth. 2. At birth the maximum rate of rise of tension in a tetanus, a measure of speed of contraction, is slow; it begins to increase only after the 5th day and reaches adult values by 21 days. 3. The increase in actomyosin ATPase activity of the rat EDL correlates closely (P ≪ 0·001) with the changes in the maximum rate of rise during the same period of development. 4. The relaxation phase of a tetanic response begins to increase in speed promptly after birth, and reaches adult values by the 21st day. The rate of calcium accumulation by the isolated sarcoplasmic reticulum (SR) increases with a similar time course. 5. The separate contributions of contraction and relaxation mechanisms to the changes in the more conventional isometric properties of maturing muscles have been analysed.
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