Obesity was produced in mice and rats with high-fat diets containing various vitamin B levels (thiamin, riboflavin, pyridoxine, Ca-pantothenate, and niacin). Enlargement of fat depots, when compared with control animals fed a low fat diet, is a function of increasing quantities of these vitamins. The variations of fat storage proceeded by increasing the fat cell number in the perirenal adipose tissue of the rat. There was no difference in the resting energy expenditure of rats fed on the diets tested regardless ofwhether the obesity was nutritionally induced or not.