The R. Q. of respiring, surviving slices of rat liver was detd. in the Warburg apparatus, in the presence of saturated and unsaturated fatty acids with the C Nos. of 16 and 18. The rats were fasted 24 hrs. before death, and 0.2 g. slices of liver were used for a detn. The solns. used were 2.5 ml. M/15 phosphate buffer, pH 7.4 and 0.5 ml. 50% H2SO4. Weighed amts. of the fatty acids were mixed with a very small amt. of 10% NaOH, emulsified with phosphate buffer and warmed to 50[degree]. The conc. of fatty acids in the emulsions was usually M/100 or M/200 and 0.5 ml. was used for a detn. Palmitic, stearic, oleic, linoleic and linolenic acids were studied. The avg. R. Q. of the control expts. without the acids was 0.89. Addition of palmitic, stearic and oleic acids decreased the R. Q. respectively to 0.60, 0.45, and 0.49. Linoleic acid decreased the R. Q. to 0.54, but linoleic acid together with oleic acid gave an R. Q. of 0.95, which is approximately that of carbohydrate combustion. NH4Cl (10 mg.), NH4Cl (10 mg.)+linoleic acid, and NH4Cl (10 mg.) + linolenic acid, gave respectively, R. Q''s. of 0.47, 0.83, and 0.51. Linolenic acid alone increased the R. Q. to 1.23.