In this study, the citrinin (CTN)-induced accumulation of reactive oxygen species (ROS) and the regulation of the activities of antioxidant enzymes were investigated in acute toxicity tests in Schizosaccharomyces pombe. 30% of the CTN was accumulated by the cells in 1000 μM CTN solution. In comparison with the control, exposure of 10(7) cells ml(-1) to 1000 μM CTN for 60 min at pH = 4.5 induced significantly (p < 1%) elevated levels of peroxides and total ROS, but not of superoxide or hydroxyl radicals, while there was a 3-fold increase in the concentration of glutathione. ROS-induced adaptation processes at cell and molecular levels via activation of the redox-sensitive transcription factors Pap1 and (in part) Atf1 resulted in significantly increased specific activities of glutathione peroxidases, glucose-6-phosphate dehydrogenase and glutathione S-transferase and in decreased levels of catalase and glutathione reductase, but no changes were detected in the activities of superoxide dismutases. This treatment caused a G2/M cell cycle arrest and elevated the number of fragmented nuclei, which is one of the markers of apoptosis. Comparison of these results with those for the positive control, 200 μM H2O2, suggested that CTN induced a medium level of oxidative stress.