3C nuclear magnetic resonance spectroscopy was used to study the biosynthesis and turnover of 0-a-D-glucopyranosyl-( 1+2)-glycerol (glucosylglycerol) in the marine cyanobacterium Synechococcus NlOO during osmotic shock. There was a rapid increase in the level of this heteroside following hyperosmotic shock, with most of the solute being synthesized from extracellular carbon via photosynthetic C 0 2 fixation. However, changes in the 3C labelling patterns during the time course of the response suggested that up to 10% of the newly synthesized material may be derived from intracellular carbon. No increase in glucosylglycerol was observed when cells were subjected to hyperosmotic shock in the dark. These experiments indicated that a good yield of highly 3C-enriched glucosylglycerol, which could serve as a source of labelled glucose and glycerol, could be obtained simply and quickly by subjecting this cyanobacterium to hyperosmotic stress in the presence of 90% 3C-enriched bicarbonate.