Dogs were given a standard diet consisting of a ground mixture of 310 g. of lean horse meat and 310 g. of cooked potatoes. The fats to be tested were melted and mixed with the diet. With natural fats, such as beef tallow, coconut oil and peanut oil, about 0.1 g-0.4 g. of ether-sol. acid substance were recovered in the urine/100 g. of fat ingested/day for 3 days. With synthetic fat mixtures of even and odd no. of fatty acids (C10-C23), the ether sol. material in the urine increased to 0.8-2.9 g./day. These fats are contaminated with branched fatty acids and dicarboxylic acids (C10-C14) which are incompletely metabolized and appear in the urine as lower acids (C6-C10). A series of new branched ethyl- and methyl-fatty acids was prepared. Fats containing straight chain fatty acids, both even and odd numbers caused no greater urinary excretion of acid metabolic end-products than natural fats. The triglycerides of the methyl branched fatty acids, [alpha]-, [beta]- and [delta]-methyldo-decanoic acid, caused only slight increase in such products, but the triglycerides of the ethyl branched fatty acids, [gamma]-ethyloctanoic acid, [delta]-ethylnonanoic acid and [delta]-ethylde-canoic acid, caused a considerable increase compared with natural fats or triglycerides with even number fatty acids. After the feeding of coconut oil, suberic and sebacic acids were isolated from the urine in pure form; after fats containing certain dicarboxylic acids, adipic, pimelic, suberic, azelaic and sebacic acids were found in the urine.