Hepatic microsomes obtained from male Sprague-Dawley-rats were used to determine the time course of the chemiluminescence (CL) resulting from the addition of three different concentrations of NADPH. At 15 to 30 minutes after the addition of NADPH, the response became maximal and then gradually diminished over the next 4 to 5 hours. Usually the higher the concentration of NADPH given, the longer time required for CL to become maximal and then disappear. The relationship between maximal, or peak, ascorbate induced CL and lipid peroxidation obtained from experimentation indicated that maximal ascorbate induced CL was related directly to the formation of malonaldehyde. In microsomal preparations where the enzymes were inactivated by heating, NADPH induced CL was eliminated, as was lipid peroxidation (LP), but no effect was noted on ascorbate induced CL or LP. Nonenzymatic, ascorbate induced CL was related directly to malonaldehyde formation in both normal and heat treated microsomes. According to the authors, the close association demonstrated between CL and LP in these studies can probably be attributed to the role of singlet oxygen in both processes.