The distribution of the branched-chain fatty acid (BCFA) was studied in skin surface lipids of laboratory animals (rat, mouse, hamster and rabbit) and the experimental animal for the study of the metabolic fate of BCFA was chosen. 2. The monoester fraction resistant to microbial degradation was the index of which fatty acids were identified and the compositions were analyzed by capillary gas chromatography and mass spectrometry (GC-MS). 3. The contents of monoester fractions in rat, mouse, hamster and rabbit were 78.9, 15.9, 30.4 and 45.6% of the total skin surface lipid, respectively. 4. BCFAs were exclusively identified to be either iso- or anteiso-series type by means of mass spectrometry. 5. The sum of iso- and anteiso-acid was the highest in hamster comprising 53% in male and 38% in female of the monoester acid. 6. Sex related differences of BCFA concentration were not so evident in the other three species. 7. BCFA contents in the monoester fractions were as follows: rat 32%, mouse 25% and rabbit 3%. 8. Concentrations of iso-series fatty acid were consistently higher than that of anteiso-series type in all animals studied. 9. Abundance of 2-hydroxy fatty acid in the rabbit monoester fraction was noted for the first time. 10. Suitability of these laboratory animals for the study of BCFA metabolism was discussed.
