Four mutants of Neurospora crassa have been isolated which have altered regulation of nitrate reductase. They each carry a mutation which results in derepressed synthesis of nitrate reductase even in the presence of glutamine. They map to a single locus which has been designated nmr-1 and which is located between am and gln on linkage group VR. The mutations appear to affect only nitrate assimilation. The nit-2, nit-3 and nit-4/5 mutations are epistatic to nmr-1 since the double mutants have the single nit mutant phenotype. For nitrate reductase synthesis, the nmr-1 mutation is epistatic to am such that the double mutant is derepressed even in the presence of glutamate or glutamine. In all other respects however, the double mutant exhibits the am phenotype. We suggest therefore that the nmr-1 mutations do not directly affect the regulation of nitrate reductase at the level of transcription but instead act post-transcriptionally.