Male albino rats of the Wistar Institute strain with an average beginning weight of 250 gm were exercised from 5–8 weeks on a training regimen consisting of swimming for 1/2 hour daily. Pair-fed, nonswimming animals served as controls. All exercised animals gained less body weight than did their controls. The adrenals and heart ventricles of the exercised animals hypertrophied significantly ( P < 0.01), while the skeletal muscles (gastrocnemii) did not. Succinic dehydrogenase activities (reduction of ferricytochrome c) were determined in the heart ventricles and the gastrocnemii. The results were expressed in terms of unit activities, actual total activities and relative total activities. The exercise, whether for 5, 6, 7 or 8 weeks, did not significantly alter the unit and actual total activities of either tissue, or the relative total activities of skeletal muscles, but did significantly increase the relative total activities of the heart ventricle ( P < 0.02).

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