The formation of the products of microsomal metabolism of the cyclic nitrosamine, nitrosohexamethyleneimine (NO-HEX) were studied. Information on the origins of the oxygen atoms in four major metabolites of NO-HEX was obtained by metabolizing this compound in an 18O2 atmosphere using microsomes and cytosol, beta- and gamma-Hydroxy-NO-HEX are formed as a result of the insertion of a hydroxyl group derived from molecular oxygen into NO-HEX. All of the oxygen atoms in epsilon-aminocaproate (EAC) were derived from water. Approximately half of the molecules of epsilon- hydroxycaproate ( EHC ) contain an 18O atom; thus, half of the alpha-hydroxy-NO-HEX formed incorporates a hydroxyl group derived from molecular oxygen with the remainder of the hydroxyls being from water. To account for the above data and the related metabolic origins of EAC and EHC ( Hecker and McClusky , Cancer Res., 42 (1982) 59; Hecker et al., Teratogen. Carcinogen. Mutagen (1982) in press), we have proposed a mechanism for the formation of these compounds from cyclic nitrosamines catalyzed by microsomal and cytosolic enzymes.