N.m.r. spectroscopy demonstrates that N-nitroso(2-hydroxypropyl)(2-oxopropyl)amine (HPOP) exists as a mixture of four isomers, A, B, C and D, the equilibrium ratios of which are 57:8:16:19, respectively, at 25 degrees C. Two of these isomers, A and B, are rotomers of the open chain conformer, while the other two, C and D, are rotomers of the ring tautomer of HPOP and are derived from A and B, respectively, via an intramolecular cyclization reaction. A syn orientation of the nitroso and carbonyl groups favors an open chain configuration (isomer A), while an anti orientation favors cyclization of the molecule (isomer D). Two forms of HPOP (I and II) which are mixtures of isomers A and C, and D and B, respectively were separated chromatographically. These two forms interconvert to each other. The first rate order constants for the interconversion reactions were determined to be 4.7 X 10(-3) and 12.8 X 10(-3)/min, respectively. During these reactions isomers A and D interconvert via the intermediate formation of isomer C. This suggests that rotomerization of C and D is thermodynamically more favorable than rotomerization of their open-chain tautomers A and B, and suggests an intramolecular interaction between the carbonyl and nitroso groups. Isomers A and D are formed during the metabolism of N-nitrosobis(2-oxopropyl)amine (BOP) and cis N-nitroso-2,6-dimethylmorpholine (NNDM), respectively, by hamster liver microsomes and NADH or NADPH. The stereo-specificity of reduction of BOP and the hydroxylation of cis NNDM results in the formation of two slowly interconvertible isomers of HPOP. This, in combination with a possible different metabolic fate of the cyclic and open tautomers of this compound, may have a significant impact on the mechanism of activation of pancreatropic nitrosamines which share HPOP as a common metabolite.