Isopentenyl diphosphate (IPP), the building block of isoprenoids, the most numerous class of secondary metabolites, is derived in plants from two pathways operating in parallel: the cytoplasmic mevalonate (MVA) and the plastidic methylerythritol phosphate (MEP) pathways. The cooperation of the two pathways (often called cross talk) is considered essential for plant adaptive responses to biotic and abiotic stresses. Elucidation of the isoprenoid metabolic pathways is indispensable for the rational design of plant and microbial systems for the production of industrially valuable terpenoids [1]. Numerous studies document that upon standard growth conditions majority of isoprenoids are made nearly exclusively via a single pathway, e.g. carotenoids and chlorophyll phytyl chains on the one hand and phytosterols on the other are derived from the MEP and MVA, respectively, while some other isoprenoids, including dolichols, are of mixed origin [2]. We have established methods, based on numerical modeling of mass spectra of metabolically labeled dolichols [3] and polyprenols, designed to quantitatively follow the cooperation of MVA and MEP in Arabidopsis thaliana. Our data indicate that in contrast to dolichols, which are ‘mosaic’ isoprenoids, polyprenols are predominantly derived from the MEP pathway. Interestingly, the relative contribution of both pathways to dolichol biosynthesis is reprogrammed upon environmental stress. Biological consequences of these observations will be discussed. This contribution is dedicated to the memory of Professor Tadeusz Chojnacki, Founder of dolichology in IBB PAS and to the memory of Professor Pavel Vlad with thanks for the cooperation.