Lipoxygenase from rabbit reticulocytes cause disruption of mitochondrial membranes and peroxidation of their lipids as judged by electronmicroscopy, release of matrix enzymes and formation of malonyldialdehyde. Without substrate mitochondria become orthodox and strong lysis by lipoxygenase appears. The lysis is prevented by ATP or ADP plus succinate; in this case mitochondria remain condensed or partly condensed. The protection by substrate was even observed in the presence of 2,4-DNP, although the mitochondria were transformed to the condensed state. Lysis was more pronounced in hypotonic than in hypertonic sucrose, condensed mitochondria are also attacked. No relation seems to exist between lipoxygenase attack and the conformational state of mitochondria. Lysis of mitochondria is dependent on the susceptibility of the fatty acid moiety of phospholipids, which may be influenced by both metabolic and structural events via alteration of protein-lipid interactions.