The disposition of N--14CH3-labeled and C-6--3H-labeled morphines in mouse tissue was determined over 48 hr after sc injection. 3H radioactivity in tissues decreased more rapidly than 14C radioactivity, and 14C activity at 48 hr after injection was 2 to 3 times greater than 3H activity in brain, blood and liver. Only 14C radioactivity in the brain and other tissues accumulated significantly by repeated co-administrations of N--14CH3- and C-6--3H-morphines. However, 3H radioactivity did not show much accumulation in brain. The ratio of 14C radioactivity irreversibly bound to macromolecules (insoluble in HCl-methanol) to the total radioactivity increased with time in the liver. [14C]Morphinone-cysteine conjugate was detected in proteolytic digests of mouse liver protein dosed with radiolabeled morphine. Morphinone-glutathione conjugate was also detected in an incubation mixture of morphine and cytosol fraction of mouse liver. These results seem to indicate that morphine is metabolized to morphinone, which then binds covalently to the thiol group of cysteine residues in protein.