Blood samples from rats and hamsters exposed to automotive engine exhausts in the Committee of Common Market Automobile Constructors long-term inhalation study at Battelle-Geneva were analysed for the levels of 2-hydroxyethylvaline (HOEtVal) and 2-hydroxypropylvaline (HOPrVal) in hemoglobin (Hb). These adducts to the N-terminus of the Hb chains were determined by gas chromatography-mass spectrometry of derivatives obtained by a modified Edman degradation that specifically cleaves off alkylated N-terminal amino acids (valine in Hb). The adduct levels found correspond to the metabolic conversion of about 5-10% of inhaled ethene and propene to ethylene oxide and propylene oxide, respectively, in agreement with results from earlier studies on mice inhaling radio-labelled alkenes. It is concluded that the alkenes, via epoxides, are the main sources of the observed HOEtVal and HOPrVal. From calculated doses and estimates of genotoxic potency the contribution from ethene in urban air to human cancer risk is discussed.