-methylpiperidine, N-methylammonium, hydrallazine or pethidinic acid.4. Several psychotropic drugs were all found to be potent competitive inhibitors of pethidine N-demethylase. These included monoaminoxidase inhibitors (the most active being nialamide and phenoxypropazine [K(i)=0.01 mM]; the least active iproniazid [K(i)=1.05 mM]); the tranquillizers promazine, propiomazine and chlorpromazine and tricyclic