The oxygenation of different types of biological membranes (rat liver mitochondria, rat liver endoplasmic membranes, inside-out erythrocyte ghosts, right side-out erythrocyte ghosts) was studied with respect to products formed during the reaction. In all cases a very similar product pattern was observed with 15S-hydroperoxy-5Z.8Z.11Z.13E-eicosatetraenoic acid (15-HETE) and 13S-hydroperoxy-9Z,11E-octadecadienoic acid, (13-HODE) being the major products. Comparison of the amount of lipoxygenase products formed with the oxygen uptake measured during the reaction indicated an excessive oxygen uptake. With mitochondrial membranes the oxygen consumption was almost one order of magnitude higher than the amount of the products detected. The origin of the excessive oxygen uptake remains unclear. These data, however, indicate that the oxygen consumption with complex substrates is not a reliable measure for the lipoxygenase activity.