Phenelzine is a suicide monoamine oxidase (MAO) inhibitor with antidepressant properties. The present study compares the inhibition of rat liver mitochondrial MAO by phenelzine and 1,1-dideuterated phenelzine and the metabolism of these drugs by that enzyme. Phenylacetaldehyde, which was measured by a high performance liquid chromatographic procedure, was found to be the major metabolite of phenelzine after incubation with MAO. The time-courses of aldehyde formation were non-linear due to the time-dependent inhibition of MAO. The reaction rate was reduced substantially when the hydrogen atom in the 1-carbon position was replaced by deuterium. The VH/VD value was 3.1, indicating a primary isotope effect. Such a substitution of deuterium in the phenelzine molecule did not affect significantly the initial reversible inhibition of MAO, which was determined by comparison of their Ki values. The irreversible inhibition, as estimated from IC50 values, however, was potentiated substantially by deuteration. These results support the notion that the irreversible inhibition of MAO activity by phenelzine proceeds via a phenylethyldiazene intermediate, which reacts with the enzyme to form a covalent adduct. An alternative pathway involving hydrogen abstraction from carbon-1 of phenelzine or via rearrangement of the diazine on the enzyme surface could occur to form a phenylethylidene hydrazine intermediate which would subsequently be hydrolyzed to phenylacetaldehyde. The reduction in the rate of phenylethylidene hydrazine formation due to the isotope effect could lead to the accumulation of phenylethyldiazene intermediate and thus potentiate the inhibition of MAO activity.