When rats were fed diets of mineralized skimmed milk powder and vitamin-fortified fats at a level of 60% of the ad libitum intake over a period of 9 weeks following weaning, the rats of growth was identical irrespective of whether the fat employed was a butter, a margarine, a commercial hydrogenated fat, or corn, cottonseed, peant or soy bean oil. Moreover, there is no indication that differences exist in the ability of the rats to respond with increased growth during a 3-week period of ad libitum feeding following the period of restricted food intake in any dietary groups. When growth hormone was injected, the augmented growth was as great or greater with the rats receiving the vegetable fat diets as with those receiving the butter diet. Since not only does increased growth not occur when growth hormone is injected in rats receiving deficient diets (i.e., vitamin Afree) but also the period of survival is decreased, the present results are interpreted as indicating that the various vegetable fats and margarine have an ability equal to butter in supporting such added growth requirements. When growth hormone was injected, a greater efficiency in the utilization of the foodstuffs for growth was found.