Mice were rendered tolerant to and dependent on morphine via a morphine pellet implantation. Three days later methadone hydrochloride was administered at a dose of 100mg/kg per os 3 hours after pellet removal and then daily for a total of 5--6 days. This dose of methadone was shown to exhibit a high efficacy for the blockade of morphine abrupt withdrawal jumping and only minimal toxicity. Under these conditions, the level of analgetic tolerance with respect to morphine and methadone and the level of dependence as measured by the naloxone ED50 were initially elevated by the morphine treatment. However, upon substitution with oral methadone these levels declined with time at a rate which did not differ from that of a group of mice receiving only water after morphine pellet removal. Despite these findings, the methadone treatment was associated with an increasing tolerance to methadone lethality during the administration of this narcotic which was nearly double that of a similarly treated water control group by the sixth day. This observation could not be explained by an elevation in the level of cellular tolerance rendered by the methadone treatment since the morphine LD50 was not elevated following identical treatment with morphine and then methadone. The significance of these results is discussed with respect to the role of methadone administration and its metabolism in the modification of tolerance and dependence.