The analgesic action of morphine-n-oxide.

1. The analgesic activity of morphine-N-oxide in mice and rats has been investigated and compared with that of morphine.2. Both morphine and morphine-N-oxide were more active when given subcutaneously than when given intraperitoneally.3. Given subcutaneously, morphine was 11-22 times more potent than morphine-N-oxide and when given intraperitoneally it was 39-89 times more potent. The potencies depended on the test situation and the species of animal used.4. In animals pretreated with amiphenazole or tacrine, the analgesic activities of morphine and morphine-N-oxide were increased. The potencies of these analgesic drugs given intraperitoneally were increased to a greater extent than were the potencies obtained by subcutaneous administration.5. A possible explanation for the increase in analgesic potency of morphine-N-oxide produced by pretreatment with amiphenazole or tacrine may be that morphine-N-oxide is rapidly inactivated in the liver and this inactivation is impaired by amiphenazole and tacrine.