Glyoxalase-I activity plays an important role in glucose metabolism and has been reported to be depressed in mice fed low levels of aflatoxin B1. In the present study examination of glyoxalase-I activity, glucose tolerance and pancreatic beta cell sensitivity was made in mice fed 0.045 ng aflatoxin B1 + 0.450 ng aflatoxin G1/g feed prenatally and for 6 mo after birth. After glucose challenge the ratios between 0-h and 2-h serum glucose levels were significantly higher than controls, indicating an increase in tolerance of glucose in the aflatoxin-fed mice with lower glyoxalase-I activity. Pancreatic beta cell sensitivity to stimulation by tolbutamide was similar in both groups. However, liver malonic dialdehyde was significantly higher in the aflatoxin-fed mice, suggesting that the altered tolerance for glucose in the aflatoxin-fed mice might be a consequence of aflatoxin mediated peroxidative actions in the liver.