Cross-tolerance to metkephamid (LY127623) produced by morphine solution ingestion by mice.

The effects of morphine and metkephamid were determined on the writhing test in drug-naive mice and mice that had been exposed to morphine solutions as their sole fluid source for a 5- or 7-day period. The average dose of morphine consumed was 435 mg/kg, using either 0.5 mg/ml solution for 7 days or a 1 mg/ml solution for 5 days. The ingestion of these morphine solutions produced a marked tolerance to the analgesic effects of morphine and cross-tolerance to the analgesic effects of metkephamid.