Wheat plants grown during 10 days in the absence of N were pretreated with 1.0 eq m -3 of methionine, asparagine or glutamine and/or 1.0 eq m -3 MSX 4 or 0.17 eq m -3 DON. Net NH4 + uptake was measured both in the presence or in the absence of the amino acid or enzyme inhibitor used in the pretreatment. The effect of met, ash and gin on net K + uptake was also studied using K +-depleted plants. Changes in the contents of root free NH +, asn, gln and the activities of GS, PEP-carboxylase, NAD+-GDH and NADH-GDH were determined. Net NH4 + uptake in gln and asn pretreated plants was markedly, and sometimes completely suppressed provided uptake was measured in the presence of the amides. On the other hand, the met pretreated plants absorbed only 35% less NH + than the control. When NH + uptake was measured in the absence of the amino acids, only those plants pretreated with asn showed a marked suppression of net uptake during the first 120 min. None of the 3 amino acids tested significantly inhibited K + uptake. Free NH4 ~-concentration in roots of N-starved plants increased after 4 h incubation with gin, asn or MSX in the absence of external NH +. Nevertheless, no correlation was observed between root NH + concentration and the extent of net NH4 + uptake suppression. The inhibitory effect exerted by asn decreased when it was supplied together with MSX or DON. Pretreatments with gln or asn in the absence of external NH + significantly increased the level of asn in the roots, while that of gin remained unchanged. It is concluded that asn and gln specifically suppress net NH4 + uptake in wheat, although it is not clear wether they act only from the root exterior, or through an endogenous pool exhibiting fast turn-over.