A study was made of diel changes in carbohydrate content, net gas exchange and export from leaf blades and leaf sheaths in Poa annua L. (a ruderal) and Poa×jemtlandica (Almq.) Richt. (a sub-arctic grass). Sucrose was the most abundant form of carbohydrate in the leaf blades with lesser amounts of starch and fructans also present. The proportions of fixed carbon allocated to export, storage and respiration in leaf blades were similar, and carbohydrates showed broadly similar patterns of diel fluctuation, in both species. However, the relative rates of sucrose and starch accumulation and depletion and the diel patterns of export in the leaf blades were different in the two species. Starch was the predominant form of carbohydrate in the sheath, with soluble sugars and fructans also present. Starch and soluble sugars showed diel accumulation and depletion in the sheaths of Poa annua but not in Poa×jemtlandica. The effects of extended darkness suggest that factors controlling carbohydrate metabolism in the sheath may be different to those operating in the leaf blade. The large fluxes of carbohydrate through the sheath suggest that it serves mainly as a thoroughfare for carbohydrates.