Considerable evidence has appeared recently connecting the mechanism of action of some antidepressant drugs with the inhibition of the enzymes responsible for enkephalin degradation. Imipramine in vitro inhibits the enkephalin-degrading aminopeptidase MII and interacts with the enzyme in a mixed competitive-noncompetitive manner. The present work shows that imipramine in vitro also inhibits reversibly soluble enkephalin-degrading aminopeptidase activity in rat brain. Kinetic analysis showed that this enzyme has two different binding sites for the drug, and that imipramine interacts with the enzyme in a mixed noncompetitive-acompetitive way.