Experiments with five narcotics, (morphine 0.3 mg/kg, oxymorphone 20 [mu]g/kg, levorphan 50 [mu]g/kg, meperidine 1.5 mg/kg and fentanyl 1.5 [mu]g/kg) and 3 narcotic antagonists (nalorphine 150 [mu]g/kg naloxone 5 [mu]g/kg, and levallorphan 20 [mu]g/kg were designed, to investigate whether or not the various antagonists exhibit a more pronounced effect against the pharmacological actions of their compounds, than against those of structurally less closely related narcotics. No such specificity of action was found. Naloxone appeared more effective than nalorphine or levallorphan. With all 3 antagonists there was a direct relation between the degree of narcotic-induced respiratory depression and the efficacy of the antagonist.