Kinetic parameters of Zn uptake were determined for wheat plants (Triticum aestivum cvs Excalibur and Gatcher, and Triticum turgidum conv. durum cv. Durati) pre-grown at deficient or sufficient Zn supply and with 0 or 4 mg m(-3) sulphonylurea herbicide chlorsulfuron [2-chloro-N-(((4-methoxy-6-methyl-1,3,5-triazin-2-yl)amino)carbonyl)-benzenesulphonamide]. Net Zn uptake generally showed a saturable response to increasing solution Zn concentrations; this response fitted a modified Michaelis-Menten equation incorporating the C-min term (solution concentration when net uptake is greater than or equal to 0). Zinc deficiency caused an increase in I-max (maximum net uptake rate) in the Zn-efficient genotype Excalibur (grows better than Zn-inefficient genotypes in environments with low Zn availability), but not in Zn-inefficient Durati. Zinc deficiency over a longer period (24 d) increased I-max and K-m in Excalibur plants more than over a shorter period (18 d). Exposure to chlorsulfuron only during the 6 h uptake period had no measurable effect on net Zn uptake. In Zn-deficient plants, 4 d chlorsulfuron exposure decreased Imax and increased C-min. A deleterious chlorsulfuron effect on I was more obvious in Zn-deficient plants after longer (10 d) than shorter (4 d) exposure to chlorsulfuron. The conclusion is that a greater capacity of Zn-deficient in comparison to Zn-sufficient Excalibur plants to take up Zn may be one of the mechanisms of Zn efficiency of Excalibur because Zn-inefficient Durati does not show an increase in I-max following a period of Zn deficiency. Prolonged exposure to chlorsulfuron (4 d) eliminates the capacity of Zn-deficient plants to increase I-max and net Zn uptake.