The nitrate reductase of etiolated field peas (Pisum arvenseL. cv. Century) is described. Enzyme activity is maximally extracted into a cysteine-free solution. Absolute enzyme activities, in dark-grown plants and in plants exposed for a short period to 600 μW∙cm−2of white light, and the extent of induction by light, were high in the terminal buds and declined rapidly in the basal direction along the shoot. Soluble protein distribution followed nitrate reductase activity in opposite direction to the acropetal decrease in nitrate concentration. Nitrate reductase in dark-grown terminal buds showed adaptation to the supply of nutrient nitrate concentration, and 2 h of white light also elicited an enzyme response that depended on nitrate. The effect of light is demonstrated to be largely independent of photosynthetic light utilization since 5 min of white light, followed by 115 min of darkness, led to activity increases that were about 80% of increases after 2 h of white light. Enzymologically, the nitrate reductase of dark-grown terminal buds appeared to be identical with the light-induced enzyme and also with the enzyme of green plant tissues as characterized in the literatur