The effects of riboflavin deficiency on the metabolism of N-nitrosodimethylamine [(DMN) CAS: 62-75-9] and other nitrosamines were examined in rats. After weanling rats were put on a riboflavin-deficient diet, the development of the deficiency was monitored by the growth rate and the erythrocyte glutathione reductase activation coefficient. In the riboflavin-deficient rats, the liver microsomal NADPH-cytochrome c reductase activity was lower but the cytochrome P450 content was higher than that of the control. The metabolism of DMN was dependent on the severity of the deficiency. During mild deficiency, which was observed mainly with Sprague-Dawley rats, the microsomal DMN demethylase (DMNd) activity was elevated 30-80%, but the metabolism of N-nitrosomethylbenzylamine (CAS: 937-40-6) and three other nitrosamines was slightly decreased. Dietary restriction in the pair-fed group also caused an elevation of DMNd activity above that of the ad libitum control group due to a partial fasting effect. During severe deficiency, which was observed mainly with Wistar rats, however, the metabolism of DMN, as well as the oxidation of benzo[a]pyrene, was decreased. Preincubation with flavin adenine dinucleotide and flavin mononucleotide enhanced the DMNd activity of the microsomes from riboflavin-deficient rats but not that from control rats. The results suggest that, depending on the alterations of the monooxygenase enzyme system during the development of the deficiency, riboflavin deficiencies may have different effects on the metabolism of DMN and some other carcinogens.