An antagonism between stereoisomeric amphetamines, amphetamine derivatives and other monoamine oxidase inhibitors has been investigated. The monoamine oxidase activity of isolated rat liver mitochondria was measured manometrically and the values checked by spectrophotofluorometric estimation of tyramine substrate to show that the oxygen uptake reflects the true enzyme activity under present experimental conditions. It was also found that concentrations of amphetamine and an amphetamine derivative P-1882 ( p -S-methylamphetamine), without effecting the enzyme activity, protected monoamine oxidase against inhibition by iproniazid, pheniprazine tranylcypromine, nialamide and P-1726 ( p -trifluoromethylamphetamine). Another amphetamine derivative P-1726 was found to be a more potent and more persistent inhibitor of monoamine oxidase than either amphetamine or P-1882. On the other hand such low concentrations of P-1726 had no protective effect on the inactivation caused by other inhibitors. Further, with respect to both direct inhibition and protection against other inhibitors, d -amphetamine was more active than the corresponding l -isomer.
