Wheat (Triticum aestivum cv Chinese Spring) supplied with 0.45 mM SO4 2- for 14 d with relative growth rates (RGR) of 0.22 to 0.24 d-1 was deprived of S for 7 to 8 d. There was no significant effect on RGR or leaf development (leaf 2 length was constant; leaf 3 expanded for 2-4 d; leaf 4 emerged and elongated throughout the experiment) during the S deprivation. In controls the net assimilation rate (A) closely reflected leaf ontogeny. S deprivation affected A in all leaves, particularly leaf 4, in which A remained at 8 to 10 μmol CO2 m-2 s-1, whereas in controls A rose steadily to >20 μmol CO2 m-2 s-1. In leaf 2, with a fully assembled photosynthetic system, A decreased in S-deprived plants relative to controls only at the end of the experiment. Effects on A were not due to altered stomatal conductance or leaf internal [CO2] ([C](i)); decreases in the initial slope of A/[C](i) curves indicated an effect of S deprivation on the carboxylase efficiency. Measurement of Rubisco activity and large subunit protein abundance paralleled effects on A and A/[C](i) in S-deprived leaves. Negative effects on photosynthesis in S-deprived plants are discussed in relation to mobilization of S reserves, including Rubisco, emphasizing the need for continuous S supply during vegetative growth.