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I . The effects of retinol and retinoic acid supplementation of retinol-deficient rats were studied for a variety of metabolic processes shown to be affected by retinol deficiency.2. Retinol-deficient rats were found to have decreased body-weight, liver and testes weights, a degeneration of testicular germinal cells, an increased incorporation of labelled choline into liver and testes phospholipids, an increased protein synthetic activity (in vitro) of liver ribosomes, an increased transfer-RNA methyltransferase activity in liver and a decreased activity in testes, an increased DNA content of testicular nuclei, and a decreased uptake of [SH]thymidine by testicular nuclear DNA.3. In retinol-deficient rats supplemented for 8 weeks with retinol these changes were reversed, measurements returning to control levels. 4.In retinol-deficient rats supplemented for 8 weeks with retinoic acid all changes were reversed except those in the testes.5 . Testicular signs of retinol deficiency appeared to be delayed when retinoic acid was added to the retinol-deficient diet of weanling rats. This suggests a sparing action of retinoic acid on the rat's utilization of retinol.6 . Suggestions are offered as to why retinoic acid will support growth and development but not spermatogenesis in the rat.It is well documented that either retinol or retinoic acid will support normal growth and development (Moore, 1957) but only retinol will maintain normal vision (Dowling & Wald, 1960) and reproduction (Howell, Thompson & Pitt, 1963).T o obtain more information on the biochemistry of retinol, we have studied the effectiveness of retinol and retinoic acid in alleviating or reversing physiological and biochemical changes associated with retinol deficiency. We have selected a few basic aspects of metabolism in liver and testes, organs known to be affected by retinol deficiency: body-and organ weights, phospholipid metabolism, protein synthesis, activity of transfer-RNA methyl transferases, DNA content of cells and thymidine incorporation into DNA. Changes in these measurements were studied for either liver or testes, or both organs, in groups of control, deficient, retinol-and retinoic acid-supplemented rats. Emphasis was placed on the effectiveness of retinoic acid to substitute for retinol in reversing the changes associated with retinol deficiency.

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