This experiment was carried out to evaluate the possibility of degradation of d-lactate into formate and acetaldehyde. In order to induce hyperproduction of d-lactate in rats. Donryu male albino rats were fed diets containing 0.064% 3'-methyl-4-dimethylaminoazobenzene (3'-MDAB), 4'-methyl-4-dimethylaminoazobenzene (4'-MDAB) or 2-methyl-4-dimethylaminoazobenzene (2-MDAB) for 10 weeks. During the experiment, body mass, food and water intake and volume of urine were documented. Methylglyoxal, d-lactate and formate in the urine samples were determined. On the first day of the eleventh week, methylglyoxal, d-lactate, glutathione and enzymatic activities of demethylation and glyoxalase I and II in liver were measured. Methylglyoxal, d-lactate and clinical chemistry parameters of blood plasma were also measured. The levels of methylglyoxal and d-lactate in livers of rats fed 3'-MDAB were very high, while those of 2-MDAB fed-rats and the control group were the same. The fact that glyoxalase I activity and the level of glutathione, a cofactor of glyoxalase I, were high in the livers of the 3'-MDAB-fed rats can explain the elevated levels of methylglyoxal and d-lactate in the liver. The most striking results were the elevated formate levels in the urine of rats fed 3'- and 4'-MDAB in a precancerous state. The degradation of d-lactate, an end product of the methylglyoxal bypass, into acetaldehyde and formate was suggested as a possible way to explain the results.