In addition, in agreement with the results obtained in goldfish (Gargiuloet al., 1996), the stimulus of G6PDH activity as a consequence of Cd exposure was observed, which indicated an increased supply of NADPH (OruC- and u¨ ner, 2000).Considering the resembling variation trends of GSSG content, Se-GPX and G6PDH activity as well as constant GR activity during the 10 days of exposure, it could be concluded that GSH turnover was strengthened against oxidative stress induced by a short-term Cd exposure.Nevertheless, if generation of GSSG is higher than the reduction back to GSH by GR, then GSSG accumulates and is translocated outside the cell by specific transporters to avoid NADPH exhaustion (Kaplowitz et al., 1996;Keppler et al., 1997), which is followed by a depletion of the GSH pool (Kretzschmar, 1996). Moreover, a constant synthesis of GSH due to the unchanged activity of g-GCS would be a contributor for the depletion of GSH pool under conditions of increasing consumption of GSH. In the current study, progressive depletion of GSH was detected and reached the lowest value at 10th day of exposure (Table 1).