The allocation of carbon in two montane grasses was followed using 14C labelling and gas exchange measurements. Both grasses showed similar total non-structural carbohydrate contents and net allocation of carbon. A six-compartmental model (with root and shoot each composed of soluble, storage and structural material) was used to summarize the data, to provide estimates of carbohydrate turnover rates, and to produce complete carbon budgets. Storage carbohydrate was heavily labelled and turned over rapidly, with half-times of 25 h in Festuca ovina and 15 h in Nardus stricta. This does not support the idea that storage carbohydrates are deposited only to be mobilized in response to environmental perturbations. It is suggested that the small relative growth rates in these species are not the result of inefficient use of fixed carbon, but of slow rates of photosynthesis; the respiratory costs of growth are similar to those for barley. The large flux of carbon through storage carbohydrate is suggested to ensure a readily available supply of sugars for metabolism, independent of environmental conditions.