A major group of steroidal hormones, called brassinosteroids (BRs), is indispensable for plant growth and development. Since their isolation from rapeseed (Brassica napus) pollen in 1979, many research laboratories have elucidated BR biosynthesis, perception, and signal transduction. BRs are synthesized in the cell interior, but, unlike mammalian steroids, are perceived in the apoplast through binding to the extracellular part of a cell surface-localized receptor kinase. In fact, the BR signalling pathway has become a paradigm for understanding receptor kinase function in plants. Despite this progress, knowledge of BR distribution in plants is lacking because these hormones are present in extremely low concentrations and their intracellular transport and cell exit routes are unknown. Our unpublished data suggest that BRs can travel over short distances to possibly activate BR responses in the neighbouring cells and consequently act in a paracrine fashion. With this project, we will address two outstanding questions: (i) where in the cell BRs are synthesized and (ii) how they are exported into the apoplast. We will employ a combination of multidisciplinary approaches that integrate cutting-edge microscopy, cell biology, proteomics, and chemistry to assess different aspects of BR export. Such information is vital for understanding cell type-specific BR signalling and how BRs induce distinct responses in different cell types.