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The growth characteristics and uptake capacities for NO $ − and NH % + , as well as activities of nitrogen assimilating-enzymes have been determined in Scleroderma verrucosum. Biomass, grown on agar or liquid MMN media, was higher on nitrate. When S. verrucosum was grown in the presence of equal amounts of ammonium and nitrate, uptake rate for NH % + was considerably higher than that of NO $ − . In addition, it was found that a limitation in ammonium concentration was a prerequisite for nitrate uptake. Nitrate reductase was stimulated when the fungus was transferred from ammonium to nitrate containing media, indicating that the enzyme was inducible by nitrate and repressed by ammonium. Glutamine synthetase and NAD-glutamate synthase activities were clearly detected in S. verrucosum, while the NADP-glutamate dehydrogenase was almost undetectable. This is consistent with the view that ammonium assimilation occurs through the GS\GOGAT cycle in S. verrucosum.Nitrogen availability is frequently a major factor limiting forest growth and the contribution of ectomycorrhizal fungi to the nitrogen nutrition of their host plants has been demonstrated (Bowen & Smith, 1981 ;Finlay, Frostegard & Sonnerfeldt, 1992 ;Thomson et al., 1994 ;Ge! ne! re! , 1995). In ecosystems such as boreal forests, ammonium is a major inorganic nitrogen source and nitrate is usually available at low concentrations (Adam & Attiwill, 1982). In calcareous or amended soils, however, nitrate may become an increasingly important nitrogen source and the concentration of nitrate could exceed ammonium concentration by several fold (Cle! ment, Garbaye & Le Tacon, 1977 ;Marschner, Ha$ ussling & George, 1991). The ability of the external mycelium to assimilate a wide range of inorganic and organic nitrogen compounds has been reported (Melin & Nilson, 1952, 1953Abuzinadah & Read, 1986Finlay et al., 1988Finlay et al., , 1989. In pure culture, most ectomycorrhizal fungi grow better on ammonium than on nitrate (Lundeberg, 1970 ;Abuzinadah & Read, 1988 ;Finlay, Frostegard & Sonnerfeldt, 1992). Several ectomycorrhizal fungi, however, such as Laccaria laccata (Ahmad et al., 1990), Pisolithus tinctorius (France & Reid, 1984 ;Plassard et al., 1991) and Hebeloma cylindrosporum (Scheromm, Plassard & Salsac, 1990 a, b) can easily take up and assimilate nitrate. Indeed, Sarjala (1990) reported that nitrate-reducing capacity differs among mycorrhizal species and differences between species and strains depend on the concentration of substrate available to the fungus.It is generally accepted that ectomycorrhizal fungi assimilate NO $ − through the involvement of nitrate reductase (EC * Corresponding author.1;6;6;3) and nitrite reductase (EC 1;6;6;4) enzymes, while NH % + produced by both enzymes or assimilated from the external mycelium will be metabolized into glutamate and glutamine by the action of NADP-dependent glutamate dehydrogenase (NADP-GDH, EC 1;4;1;4) and glutamine synthetase (GS, EC 6;3;1;2)\glutamate synthase (EC 1;4; 1;14) (GS\GOGAT) cycle (Botton & Chalot, 1...