Nitrate reduction in vivo by spinach leaf discs was shown to be inhibited by hydroxylamine when this was included in the nitrate reductase assay solutions or introduced to the tissue during a preincubation period. The sensitivity of nitrate reduction to hydroxylamine was not sufficient to suggest a natural process, considering the small endogenous concentrations of hydroxylamine in the leaves. Inhibition of nitrate reduction in vivo could be approximately related to rates of in vitro inhibition of nitrate reductase by this compound. There was no need to suppose conversion of hydroxylamine to cyanide to inhibit nitrate reduction. Some of the in vivo and in vitro characteristics of hydroxylamine inhibition of nitrate reductase are described. Hydroxylamine was metabolised by discs at rates comparable to nitrate reduction. Rates of metabolism of hydroxylamine, and its accumulation in the tissues from an external solution were both enhanced by light but little affected by anaerobiosis.