Abstract Mixtures of morphine-6 3H and morphine-N- 14CH 3 were incubated with rat brain subcellular fractions. Isotope ratio measurements served as the marker for identification, purification and quantitation of N-nor products which were shown to consist almost solely of N-normorphine. The microsomal, synaptosomal and mitochondrial, but not the supernatant brain preparations yielded N-normorphine. The microsomal incubations were then repeated in the presence of cytochrome P-450 inhibitors which suppressed the liver reaction but did not affect the brain biotransformation. The brain N-dealkylase is therefore different from the one in the liver and is not a cytochrome P-450 linked enzyme.