Blood samples were obtained from male Fischer 344 rats exposed to controlled air concentrations of ethylene oxide; 0, 10, 33, and 100 ppm, 6 h/day, 5 days/week, for 2 years. N tau-(2-hydroxyethyl)histidine was isolated from hemoglobin hydrolysates and analyzed quantitatively by means of gas chromatography--mass fragmentography and by amino acid analysis. The degrees of alkylation found were 1.3 and 2.8 nmol hydroxyethylhistidine per gram hemoglobin in two groups of unexposed rats, and 14, 34, and 82 nmol per gram hemoglobin, respectively, at the three air levels of ethylene oxide. Rats of the same breed were given two concentrations of radiolabeled ethylene oxide by IP injection. The degrees of alkylation of amino acids in hemoglobin and of guanine-N-7 in DNA from livers and testes were determined. The degrees of alkylation of liver and testicular DNA were about 150% and 50%, respectively, of the values expected from the degree of alkylation of hemoglobin, basing the expectancy on a direct proportionality between the reactivity of the specific nucleophilic sites and the degree of alkylation obtained at these sites, assuming that the dose of ethylene oxide was the same in the different tissues studied. The in vivo dose of ethylene oxide determined from data on hemoglobin alkylation thus gives a reasonable approximation of the DNA dose. The data were in agreement with a fast elimination of ethylene oxide from the tissues, the biological half-life being estimated as about 10 min.