1. The growth potential of normal rat liver was tested by means of the tissue culture method. Under the experimental conditions used, the growth potential was found to be an inverse proportional function of the age of the animal.

  2. The growth potential of regenerating rat liver was found to be independent of the age of the animal and of the same order of magnitude as that of the normal liver of young animals.

  3. The restorative effect of regeneration on the growth potential of liver of old animals outlasted the regenerative process itself and was still apparent 1 month after the completion of regeneration.

  4. Differences in the morphology and organization of the colonies of normal liver at different ages and of regenerating liver were described and their relationship to growth potential considered.

  5. The significance of these results with respect to the general physiology of growth and carcinogenesis was discussed.

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