The biosynthesis of the monoterpenes limonene and carvone in the fruit of caraway (Carum carvi L.) proceeds from geranyl diphosphate via a three-step pathway. First, geranyl diphosphate is cyclized to (+)-limonene by a monoterpene synthase. Second, this intermediate is stored in the essential oil ducts without further metabolism or is converted by limonene-6-hydroxylase to (+)-trans-carveol. Third, (+)-trans-carveol is oxidized by a dehydrogenase to (+)-carvone. To investigate the regulation of monoterpene formation in caraway, we measured the time course of limonene and carvone accumulation during fruit development and compared it with monoterpene biosynthesis from [U-14C]Suc and the changes in the activities of the three enzymes. The activities of the enzymes explain the profiles of monoterpene accumulation quite well, with limonene-6-hydroxylase playing a pivotal role in controlling the nature of the end product. In the youngest stages, when limonene-6-hydroxylase is undetectable, only limonene was accumulating in appreciable levels. The appearance of limonene-6-hydroxylase correlates closely with the onset of carvone accumulation. At later stages of fruit development, the activities of all three enzymes declined to low levels. Although this correlates closely with a decrease in monoterpene accumulation, the latter may also be the result of competition with other pathways for substrate.