Barley (Hordeum vulgare L. cv. Agneta) was cultivated hydroponically under continuous light, constant temperature and relative humidity. The relative growth rate (RGR) was kept at 25% by an exponential supply of complete medium for 2 wk. The cultures were then transferred to phosphorus-free media and the amounts of total P, fructans, starch, sucrose, glucose and fructose in sink and source leaves were measured at 0, 12, 24, 48, 72, 120 and 156 h. The capacity of net fructan synthesis (i.e. total activity of sucrose[ratio ]sucrose 1-fructosyltransferase (1-SST) and sucrose[ratio ]fructan 6-fructosyltransferase (6-SFT)), the activity of fructan exohydrolase (FEH), as well as the activity of acid invertase were also measured in the two types of leaves. Omission of P from the growth medium caused a decrease in the P content in both sink and source leaves, whereas the RGR was not affected during the 156 h of the experiment. The fructan concentration increased in low-P plants whilst there was little or no effect on the concentrations of starch, sucrose, glucose and fructose. There was a slight increase in the capacity of net fructan synthesis in both sink and source leaves at 156 h after the onset of P-starvation. There was no change in the activity of FEH or acid invertase. The results indicate that in barley leaves the mechanism for carbon partitioning into fructans is more sensitive towards low P conditions than the mechanism for carbon partitioning into starch.