Abstract.In the preliminary purification ofCapsicumleaf nitrate reductase (EC 1.6.6.1), treatment of the crude extract on Sephadex G‐25 was necessary to prevent a gelling of the extract and sedimentation of the enzyme. Its Kmvalues for NADH and nitrate were estimated to be 9.3 and 105mmol m−3ADP and ATP gave hyperbolic competitive inhibition, with respect to NADH, while the inhibition by AMP was linear competitive. Kivalues calculated were: ADP and ATP approximately lmol m−3and AMP 2.3 mol m−3. Inhibition by ADP was not altered by reduced glutathione.TheCapsicumnitrate reduclase was very susceptible to inhibition by NADH (in the absence of nitrate) and anin vivoassay showed that the activity of the enzyme was limited by the supply of nitrate. NADH and adenine nucleotide levels measured in theCapsicumleaf were used to estimate inhibition of nitrate reductase and a prediction was made of the nitrate reductase activity at different times in the photoperiod. This was shown to follow the same trend as the measuredin vivoactivity of the enzyme. Changes in adenine nucleotide levels had little effect on nitrate reductase a