On a vitamin A-free diet the life span of albino rats increases with increasing vitamin A intake at levels of 1, 2, 4 and 20 U.S.P. vitamin A units per 100 gm. of body weight. The effect of increasing vitamin A levels is reflected similarly on the four physiological criteria studied — life span, growth, tooth structure, and eye condition. The vitamin A requirement of the albino rat estimated on the basis of maximum life span and optimal growth may be approximated at 10 U.S.P. units of vitamin A per 100 gm. of body weight. This is about five times the minimum requirement previously estimated on the basis of vaginal smear records.