AbstractDuring investigations on theN‐oxygenation of adenine (1) the enzymatic formation of adenine 1‐N‐oxide3was demonstrated for the first time. The identity of this metabolite was confirmed by its chromatographic behaviour and UV‐spectrum recorded after HPLC separation. Adenine 1‐N‐oxide (3) and similar oxygenated derivatives of adenine were synthesized as reference substances. The enzymatic formation of3exhibits the typical characteristics of a reaction catalysed by microsomal mono‐oxygenases. In induction experiments, an increase in the rate of formation of3after pretreatment with phenobarbital was observed. A participation of those isoenzymes of the cytochrome P‐450 enzyme system which can be induced by phenobarb