This study was conducted to determine the single and combined effects of cadmium (Cd) exposure at different concentrations and durations on biological responses in Gammarus pulex. The animals were exposed to 4, 8, 16 or 32 mu g l(-1) of CdCl2 for 24, 48, 72 or 96 h, respectively. Cd accumulation, protein content, malondialdehyde content (MDA) and three antioxidant enzymes superoxide dismutase (SOD), catalase (CAT) and glutathione peroxidase (GPx) were studied. The results were analyzed with two-way analysis of variance (two-way ANOVA) to assess the significance of the effects of exposure concentration and duration and their combined effects. The highest accumulation of Cd was found as 15.07 mu g g(-1) at 32 mu g l(-1) and 96-h Cd exposure. There was a significant correlation between accumulated Cd concentration and MDA and protein content and GPx activity. Although there was no significant synergistic effect of duration and concentration on Cd accumulation, a significant synergy of duration and concentration was observed in all other studied parameters. Interestingly, with the exception of GPx activity, the effect of concentration was greater than the effect of duration for all studied parameters. GPx activity showed that duration had a greater effect than concentration. Additionally, both SOD and GPx are more effective than CAT during Cd stress. The results of this study may be useful for understanding the relationship between organisms and environmental toxins.