Liver microsomes from uninduced hogs catalyzed the NADPH-dependent N-oxidation of the hydroxylamine (Z)-3-(4-bromophenyl)-N-hydroxy-N-methyl-3-(3-pyridyl)allylamine. The conjugated nitrone N-[(2Z)-3-(4-bromophenyl)-3-(3-pyridyl)-2-propenylidene]methylamin e N-oxide was the principal product and accounted for 90-95% of the total hydroxylamine metabolized by microsomes. The nitrone had a relatively high chemical stability which was investigated at various conditions. Studies of the biochemical mechanisms for N-oxidation of N-hydroxy-norzimeldine showed that the reaction was catalyzed by the flavin-containing monooxygenase. This conclusion was based on studies with the purified hog liver flavin-containing monooxygenase and hog liver microsomes. Purified rat liver cytochrome P-450IIB-1 was ineffective at catalyzing N-oxidation of N-hydroxynorzimeldine.