An application of statistical method to the data of 469 vitamin A feeding tests was made with a view to determining the factors influencing the weight gains of the test animals. The data of this study indicated a possible difference in the reactions of rats fed vitamin A free diets containing different amounts of fat. The data showed also that the quantity of basal diet ingested was the measured variable having the greatest per cent effect upon the weight gains of the animals;An experiment was conducted to test the difference in the reactions during the depletion period of vitamin A test-animals fed diets containing and not containing fat. One hundred twenty-three animals were fed the basal diet containing fat and 60 the fat-free basal diet. The difference in the mean gains in weight was not found to be significant but that between the number of days to depletion was significant. From these data one may conclude that a vitamin A free diet containing fat is preferable to a fat free basal diet as it shortens the time required to deplete the body store of vitamin A;An analysis of the reactions of vitamin A test animals from three different stock colonies revealed significant differences showing that the results of this or any other study could not be applied directly to the animals of any other colony without first testing the homogeneity of the two colonies;Since the food intake was shown to be the factor having the greatest percent effect upon the gains in weight of the test animals, an attempt was made to regulate their gains by controlling this factor. An estimate of the food intake was made by means of a regression equation in which the mean initial weight, mean gain and mean days to depletion were used as the independent variables and mean daily food intake as the dependent variable. Five groups of 20 rats each were fed a quantity of the basal diet estimated by the regression equation. This method of feeding the animals did not result in less variable weight-gains. In analyzing the data to discern reasons for the failure of these 100 animals to react positively to the test it was discovered that the test animals and the 123 animals upon which the regression was based do not belong to the same population even though both groups were offspring of a highly inbred colony of rats. (Abstract shortened by UMI.)