(1'R,2'S)-nicotine-1'-N-oxide is a stereoisomer of nicotine-1'-N-oxide, an oxidation product of S-(-)-nicotine. N-1'-oxidation of nicotine is mediated via flavin-containing monooxygenase. A marked stereoselectivity has been observed in the formation of N-1'-oxide metabolites from nicotine. Mammal hepatic preparations generally produce more the N-1'R,2'S-cis diastereoisomer than N-1'S,2'S-trans diastereoisomer after incubation with S-(-)-nicotine. cis- and trans-nicotine-N'-oxides modulate the development of tumors in rats initiated with N-[4-(5-nitro-2-furyl)-2-thiazolyl]formamide.