When the analgesic and antipyretic drug acetaminophen is exposed to the horseradish peroxidase/hydrogen peroxide system, light emission is observed. Both the rate of reaction and the emission intensity are increased in the presence of the cationic detergent CTAB. The emission spectrum indicates the presence of at least three emitters. The peak at 445 nm is close to the position of the only band observed in the fluorescence spectrum of the spent reaction mixture and is ascribed to excited N-acetyl-p-benzoquinoneimine. The other two emission bands (505 and 580 nm) must be due to transients. Total emission from the system increases in the presence of Triton X-100 solubilized-chlorophyll; the main donor to chlorophyll is the 445 nm emitter. The generation of excited species in this model system of acetaminophen peroxidative metabolization expands the number of candidates responsible for the deleterious effects of the drug in high doses.