A sensitive fluorometric assay for measurement of lipid peroxidation damage to biological preparations and tissues is described. The method consists of a chloroform-methanol extract of tissue followed by measurement of fluorescence characteristics of products derived from lipid peroxidation. Fluorescence analysis has been used successfully with rats and mice in vivo with stress induced during dietary and aging experiments and in peroxidation of subcellular organelles in vitro. Replicate extractions of fluorescent tissue showed a standard error of 2%. In a test of linearity with concentration, the amount of lipid-soluble fluorescent material in extracted samples was directly proportional to that added before extraction. Specific and general applications to in vivo and in vitro lipid peroxidation experiments are diseussed.