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