1. Retinol esterification in the small intestine, liver and kidney of rats given a normal diet or a vitamin-A-free diet and of rats given large doses of vitamin A was studied. The active enzyme is a microsomal acyl CoA: retinol acyl transferase (ARAT).
2. In the small intestine ARAT activity was 0.37 nmol ester/mg microsomal protein per min. Large doses of vitamin A increased the activity significantly, while the enzyme activity in the vitamin-A-deficient rats was in the range of that of the controls. Retinoic acid in physiological doses (0.064 mg three times per week) had no influence on ARAT activity.
3. In the liver, ARAT activity of the controls was 0.58 nmol ester/mg microsomal protein per min. The activity was increased after large doses of vitamin A. It was not significantly reduced in vitamin-A-deficient animals.
4. The kidney had a low, but significant ARAT activity, both in normal and vitamin-A-deficient animals and after large doses of vitamin A (range 0.08–0.14 nmol ester/mg microsomal protein per min).
5. The vitamin-A-esterifying enzyme in the small intestine and liver of the rat seems to be influenced by the amount of retinol in the diet.