Whole fat from chicken liver, heart, adipose tissue, and plasma was extracted with chloroform-methanol solvents. Fat content of tissues was determined, and total fat was analyzed for lipid composition by chemical assay, for malonaldehyde content by improved thiobarbituric acid (TBA) assay with antioxidant protection, and for fluorescence excitation (360 nm) and emission (440 nm) spectra. The fatty acid composition of isolated phospholipids and triglycerides from each tissue was also determined. Liver contained 5.56% fat composed of 42.3% phospholipids, 51.2% triglycerides, and 6.2% total cholesterol; heart contained 4.27 % fat composed of 47.6% phospholipids, 46.9% triglycerides, and 5.3% total cholesterol; and adipose tissue contained 79.4% fat composed of .7% phospholipids, 99.1% triglycerides, and .1% total cholesterol. Plasma contained 1.25% fat composed of 45.6% phospholipids, 31.4% triglycerides, and 22.3% total cholesterol. Compared to the fatty acids of triglycerides, the fatty acids from tissue phospholipids were much more polyunsaturated with prominent amounts of arachidonic acid. Generally, fats with large phospholipid fractions also had relatively large concentrations of malonaldehyde. The concentration of malonaldehyde in fat from adipose tissue, which contained less than 1% phospholipids, was more than 50 times lower than in fat from other tissues studied. The relative levels of fluorescent products in fat from tissues followed the pattern established for malonaldehyde levels in those tissues. Because of the widely variable fat content of different tissues, the concentration of malonaldehyde in extracted fat appeared to be a more useful parameter for evaluating potential oxidative rancidity than the TBA number.