Malonaldehyde formation by cortical brain slices from rat brain was determined as a function of incubation time and of oxygen pressure. This substance, a byproduce of lipid peroxidation, was detected by the thiobarbituric acid test. Significant amounts of malonaldehyde were formed by brain slices during incubation in the 0.2 (air) to 10 atm oxygen range, and a portion of it was released into the medium. The rate of malonaldehyde formation was the highest during the first 10 min. Elevation of oxygen pressure above 1 atm caused further increments in malonaldehyde production with kinetic properties similar to that seen at 1 atm pressure, but the increments per additional oxygen pressure were diminishing. The formation of a given amount of malonaldehyde can be expressed as a function of atm oxygen X min. This function has the shape of a saturation curve approaching a maximum at around 300 atm X min. The results indicate extensive lipid peroxidation in brain slices under standard incubation conditions.