Swiss albino mice (7-8 weeks old) were irradiated with different doses (0-25 Gy) of gamma-radiation at a dose-rate of 0.05 Gy/s. The specific activities of glyoxalase I (GI) and glyoxalase II (GII) were determined in the spleen and liver immediately and on the 3rd and 6th day postirradiation. The results indicate that the glyoxalase system is radiosensitive, particularly glyoxalase I whose activity was enhanced even at low doses (0.5 Gy). The magnitude and mode of the radiation effect depends on dose and tissue. The patterns of the GI/GII ratio in the liver and spleen was very similar when measured immediately after irradiation. The radiation effect on the glyoxalase system persists even in the postirradiation period and was inversely related to the dose-rate. GSH and caffeine increased and chlorpromazine decreased the radiation-induced activity of GI, but all three modifiers enhanced radiation-induced inactivation of GII. Since the glyoxalase system may play an important role in the regulation of cell division and differentiation, radiation effects on this system may have some biochemical consequences.