We present immunocytochemical localizations of four enzymes involved in p-menthane monoterpene biosynthesis in mint: the large and small subunits of peppermint (Mentha x piperita) geranyl diphosphate synthase, spearmint (Mentha spicata) (2)-(4S)-limonene-6-hydroxylase, peppermint (2)-trans-isopiperitenol dehydrogenase, and peppermint (1)-pulegone reductase. All were localized to the secretory cells of peltate glandular trichomes with abundant labeling corresponding to the secretory phase of gland development. Immunogold labeling of geranyl diphosphate synthase occurred within secretory cell leucoplasts, (2)-4S-limonene-6-hydroxylase labeling was associated with gland cell endoplasmic reticulum, (2)-trans-isopiperitenol dehydrogenase labeling was restricted to secretory cell mitochondria, while (1)-pulegone reductase labeling occurred only in secretory cell cytoplasm. We discuss this pathway compartmentalization in relation to possible mechanisms for the intracellular movement of monoterpene metabolites, and for monoterpene secretion into the extracellular essential oil storage cavity.Monoterpenes are a large and diverse class of volatile C 10 isoprenoids that are the major constituents of many plant essential oils and resins. These natural products play important chemoecological roles in the interactions of plants with their environments. Some monoterpenes have been implicated as allelopathic agents, and they often directly, or indirectly, protect plants from herbivores and pathogens (Pickett, 1991;Harborne, 1991;Langenheim, 1994;Wise and Croteau, 1999;Hallahan, 2000). As important constituents of floral scents, monoterpenes also function to attract pollinators (Dudareva and Pichersky, 2000). Some plants release volatile monoterpenes and sesquiterpenes in response to herbivore damage that function to attract predatory insects that in turn feed on, or parasitize, the herbivorous insects (Langenheim, 1994;Degenhardt et al., 2003). In conifer species, mechanical wounding, insect attack, or applications of methyl jasmonate can induce resin secretion and differentiation of traumatic resin ducts within wounded tissues, producing a protective barrier of resin at the site of wounding (Steele et al., 1995;Trapp and Croteau, 2001;Franceschi et al., 2002;Martin et al., 2002). Many plant species constitutively produce large quantities of terpenoid-rich resins and essential oils within specialized glandular tissues, such as glandular trichomes, secretory cavities, and secretory ducts (Fahn, 1979(Fahn, , 2000. These natural stores of plant terpenoids probably serve as deterrents to herbivorous insects, but they also provide commercially important sources of flavorings, fragrances, resins, and pharmaceuticals (Langenheim, 1994;Wise and Croteau, 1999). The glandular cells of these secretory tissues are of interest for their remarkable ability to rapidly generate substantial amounts of specific terpenoid products.Peppermint (Mentha x piperita) has been employed as a model system for the study of monoterpene biosynthesis (Wise an...