Organophosphorus pesticides are one of the most commonly used insecticide classes. They act through a potent inhibition of acetylcholinesterase (AChE). Many of them must undergo transformation into the corresponding oxon analogs to inhibit AChE. This study showed that a brain tissue subfraction transformed methyl parathion (O,O-dimethyl O-p-nitrophenyl phosphorothioate) in vitro. Methyl parathion activation was assayed by solvent extraction of the products followed by HPLC and GC-MS analyses and, indirectly, by the inhibition of AChE present in the incubation mixture. The lack of impairment of AChE after 2 h of incubation of the brain subfraction with methyl parathion and, alternatively, with NADPH, CO, SKF 525-A, piperonyl butoxide or nitrogen indicated that this brain subfraction transformed methyl parathion without the involvement of a mixed-function oxidative pathway. The results from HPLC analysis did not show a peak corresponding to methyl paraoxon (O,O-dimethyl O-p-nitrophenylphosphate), but showed the production of an unidentified peak which eluted nearby standard methyl parathion (retention times of 10.65 and 8.86 min, respectively). GC-MS analysis suggested that the unidentified product could be a methyl parathion isomer.