1. The study of the nitrate reducing system in wheat viz Triticum aestivum L., is presented. Control of the pathway is considered important and effects of factors such as ammonium, nitrate and light are investigated. 2. Assay conditions for nitrate reductase (NR) and nitrite reductase (NiR) in the same crude extracts and NR in vivo in root and leaf tissues are described. 3. NH and NiR are not directly affected by ammonium but ammonium decreases both NR activity in vivo and nitrate uptake. Possibly, competition between glutamate dehydrogenase and NR for NADH causes apparent inhibition of NR activity in assays of root tissue in vivo. h. Exogenous nitrate is necessary to maintain high NR and Ni“ activity. 5. Light induces NR and NiR in leaves and is required for high activity in both root and leaf tissues. Nitrate uptake by the leaves, but not roots is affected by light. It is suggested that the transport of nitrate from xylem vessels into leaf cells is a light requiring process. 6. NR activity in seedlings grown under apparently constant conditions is shown to undergo an endogenous, circadian rhythm with a preset phase. The quality of light is important in determining the rhythm and in.producing, high NR activity. A specific NR protease may be implicated in the regulation of the rhythm and in the rapid decrease in NR activity in seedlings subjected to darkness.