The growth of Hebeloma cylindrosporum Romagn. on nitrate medium is associated with nitrate reductase activity equivalent to that measured in herbaceous angiosperms. Nitrate is not indispensable for the induction of NR, as thalli cultivated in ammonium have the same NR activity as thalli cultured on nitrate. However, the simultaneous presence in the culture medium of ammonium with an organic acid such as malate, citrate or succinate causes a diminution of NR activity. Glutamine or asparagine, by-products in the assimilation of ammonium and malate, could intervene in the regulation of this enzyme. The data suggest that the absence of nitrogen or the presence of a nitrogen source, such as ammonium, inappropriate for growth derepresses nitrate reductase in this fungus.