(1) We investigated the distribution of morphine and morphine-6beta-glucuronide (M6G) in the brain and spinal cord after intracerebroventricular (i.c.v.) injection of each drug in rats. (2) The cerebrospinal fluid (CSF) concentration of M6G was 5-37 times greater than that of morphine 10, 60 and 120 min after the i.c.v. injection. The apparent elimination clearance of M6G from the CSF was 10 times lower than that of morphine. (3) The intrathecal CSF concentration of M6G measured by the microdialysis method was 29-79 times greater than that of morphine, and M6G was rapidly distributed into the intrathecal space after the i.c.v. injection. (4) M6G was detected in the cerebrum, brainstem, cerebellum and spinal cord at concentrations 2-21 times higher than morphine after the i.c.v. injection of each drug. The distribution volume of M6G in rat brain slices was three times lower than that of morphine, and close to the extracellular fluid space in the brain regions corresponding to the vicinity of the opioid receptors. (5) These brain distribution characteristics of M6G, namely, low clearance from the central nervous system, localization in the extracellular fluid and rapid distribution into the intrathecal space, may contribute to the potent analgesic effect of M6G after i.c.v. injection.