Metabolism of N-nitroso-2-oxopropylpropylamine (NOPPA) with either a microsomal preparation or the soluble enzyme fraction from rat liver yielded N-nitroso-2-hydroxypropylpropylamine (NHPPA). We have also identified and quantified propionaldehyde, n-propanol and isopropanol following incubation of NOPPA with the microsomal fraction.