The effects of 6 weeks feeding with diets low in methionine, choline, or proteins on the biotransformation and retention of 76As-labelled arsenite (0.4 mg As/kg bw) in rabbits have been studied. All test diets caused a significant decrease in the urinary excretion of dimethyl[76As]-arsinic acid, the main metabolite of inorganic arsenic. This gave rise to an increased retention of arsenic in the tissues, especially liver and lung. The test diets also caused a specific increase of the arsenic concentration in liver microsomes. The results indicate that subjects with a poor nutritional status have a lower capacity of methylating and thereby detoxifying inorganic arsenic.