In order to understand the oxidation cleavage of fatty acids in which the number of carbon atoms is in the range of those appearing in nature, 2-propylmyristic acid was given to rabbits and 2-propyladipic acid was recovered from their urine. Comparing this result with the works carried out by earlier researchers, a hypothesis of ζ-oxidation was proposed. However, when the rabbits were given 2-propyltridecanoic acid, from their urine was recovered a mixture of acids in 70-30 gravimetric proportions of 2-propylglutaric and 2-propylpimelic acids by which the possibility of ζ-oxidation was completely lost. It was further confirmed that branched-chain fatty acids possessing 16-17 C-atoms underwent ω-oxidation at first, then β-oxidation, finally liberating the two C-atom fragments from the end.