Changes of methylglyoxal (MG) level accompanying the changes of glyoxalase I and II activities were examined in the liver and blood of mice inoculated with L1210 leukemia or Sarcoma 180 intraperitoneally. The amounts of MG in the liver and blood decreased for the first 3-5d after the inoculation of L1210 leukemia or Sarcoma 180 and then increased significantly. Glyoxalase I and II activities, on the other hand, increased for the first 3-5d and then decreased on day 8. The levels of dihydroxyacetone phosphate, glyceraldehyde-3-phosphate and fructose-1, 6-diphosphate decreased gradually during the experimental period of 8d in the liver and blood of mice with Sarcoma 180.The present observations may suggest that the initial proliferation of the tumor cells is accompanied with a decrease of MG and further development of the tumor cells results in an inductive increase of MG level due to the acceleration of sugar catabolism and/or to the decrease of glyoxalase I and II activities.