Pethidine 3 mg/kg and pentazocine 0·5 mg/kg or 2 mg/kg were administered to subjects anaesthetized with thiopentone, nitrous oxide and oxygen. Cardiovascular and respiratory effects were studied. Pethidine 1 mg/kg and pentazocine 0·5 mg/kg were approximately equidepressant. Pethidine 1 mg/kg prevented the depressant effects of subsequently administered pentazocine. Pentazocine 2 mg/kg protected from the effects of pethidine, but pentazocine 0·5 mg gave only partial protection. Further work is necessary to delineate the interaction of pentazocine with the narcotic analgesics, particularly with reference to the analgesic effects.
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