The regulation of nitrite reductase in Lemna minor has been studied. The evidence indicates that in nitrate-fed plants nitrate itself is the inducer of nitrite reductase. The enzyme is subject to end-product repression by ammonia and various amino acids. Nitrate reductase is also repressed by a similar range of compounds. Most of the repressors tested are more effective when nitrate rather than nitrite is supplied as the inducer. The effects of cyclo-heximide, D-threo-chloramphenicol and lincomycin on the induction by nitrate and nitrite suggest that both enzymes are synthesized on cytoplasmic ribosomes. The mechanism of repression by ammonia and amino acids is discussed.