1. 1. Mitochondria from the adrenal glands of the cat and guinea-pig were isolated, their lipids extracted and separated into neutral lipids and phospholipids. Both fractions were investigated by thin-layer, column and gas chromatography. 2. 2. Neutral lipids of both species consist predominantly of cholesterol esters. While cholesterol esters from the cat contain appreciable quantities of C 2- and C 22 unsaturated fatty acids these acids are almost completely absent from the cholesterol esters of the guinea-pig. Mitochondrial triglycerides in both species contained considerable quantities of arachidonic acid. 3. 3. The main component of the phospholipids of both species was phosphatidyl choline followed by phosphatidyl ethanolamine and diphosphatidyl glycerol. 4. 4. Various phospholipids fractions differ considerably in their fatty acid composition from mitochondrial phospholipids in adrenal glands of other species. In general terms individual phospholipids had a rather similar composition as phospholipids from mitochondria in other organs. The presence of adrenic acid (C 22:4ω6) could be confirmed only in phosphatidyl ethanolamine from the guinea-pig.