Histochemical and autoradiographic techniques have been used to investigate the sites of synthesis, transport and location of sulphated polysaccharides in some larger brown seaweeds.The most rapid uptake of (35)SO4 occurred when material was incubated in medium with 10(-4)M carrier sulphate, negligable uptake occurring from seawater.Autoradiography using (35)SO4 has shown that in Pelvetia sulphated material is synthesised by all cell types, particularly epidermal cells. In Laminaria spp. this activity is confined to specialized secretory cells which discharge into mucilage canals. In both instances the process of carbohydrate sulphation appears to occur in the Golgi-rich perinuclear region.