The monoterpene ketone l-menthone is specifically converted to l-menthol and l-menthyl acetate and to d-neomenthol and d-neomenthyl-beta-d-glucoside in mature peppermint (Mentha piperita L. cv. Black Mitcham) leaves. The selectivity of product formation results from compartmentation of the menthol dehydrogenase with the acetyl transferase and that of the neomenthol dehydrogenase with the glucosyl transferase. Soluble enzyme preparations, but not particulate preparations, from mature peppermint leaves catalyzed the NADPH-dependent reduction of l-menthone to both epimeric alcohols, and the two dehydrogenases responsible for these stereospecific transformations were resolved by affinity chromatography on Mātrex Gel Red A. Both enzymes have a molecular weight of approximately 35,000, possess a K(m) for NADPH of about 2 x 10(-5)m, are very sensitive to inhibition by thiol-directed reagents, and are not readily reversible. The menthol dehydrogenase showed a pH optimum at 7.5, exhibited a K(m) for l-menthone of about 2.5 x 10(-4)m, and also reduced d-isomenthone to d-neoisomenthol. The neomenthol dehydrogenase showed a pH optimum at 7.6, exhibited a K(m) for l-menthone of about 2.2 x 10(-5)m, and also reduced d-isomenthone to d-isomenthol. These stereochemically distinct, but otherwise similar, enzymes are of key importance in determining the metabolic fate of menthone in peppermint, and they are probably typical of the class of dehydrogenases thought to be responsible for the metabolism of monoterpene ketones during plant development.