Levels of N-Methylvaline (MeVal) and N tau-methylhistidine (MeHis) were measured in male smokers and non-smokers in a program aimed at mapping background alkylations of hemoglobin (Hb) as potential indicators of doses of exogenous and endogenous genotoxic agents. MeVal was also determined in Hb from rats, Syrian golden hamsters, mice and chickens. MeVal was found to occur at levels around 0.5 nmole/g Hb, with relatively little variation between individuals and species. MeVal was not significantly affected by smoking. This result contrasts with elevated levels of N-hydroxyethylvaline (HOEtVal) measured in the same persons (Törnqvist et al., 1986b). Levels of S-methylcysteine (MeCys) (Bailey et al., 1981) and MeHis were much higher than those of MeVal. The high levels of MeCys and MeHis may be due partly to misincorporation during protein synthesis and to artifacts. S-Adenosylmethionine and formaldehyde are possible endogenous sources of MeVal. One individual (smoker) out of 21 selected for measurement of MeVal was an outlier, with raised levels of both MeVal and HOEtVal, as would be expected in case of a defective detoxification system.