Various combinations of HgCl2 and sodium selenite were administered orally to rats and mice in vivo for both chronic and acute treatment. The antagonistic interaction and the optimum combination at which the action of HgCl2 could be neutralised by Na2SeO3 was investigated. Of the concentrations employed in rats when Na2SeO3 was fed after 1 h treatment with HgCl2, the percentage of chromosomal abnormalities significantly decreased as compared to animals treated with HgCl2 alone. All other experiments with different combinations of HgCl2 and Na2SeO3 gave higher frequencies of chromosomal abnormalities as compared to Na2SeO3 alone. An acute dose of HgCl2 was lethal, but in combination with Na2SeO3 at 1 h intervals, the abnormalities decreased to 60%. The results with mice showed antagonism in mixtures at higher doses, which was not evident in lower doses.