The effects of sublethal concentrations of cadmium (0.64 µg/L), iron (0.043 mg/L) and zinc (0.31 mg/L) and a mixture of these metals on succinate dehydrogenase (SD) and alkaline phosphatase (AP) activity and on structural changes in the mitochondria of epithelium cells of the digestive tract were examined in the oligochaete Limnodrillus hoffmeisteri after 96 h of exposure in artificial sediments. SD activity was significantly inhibited, particularly in treatments with Cd alone (92.57%), while AP increased its activity with Cd alone (73.23%). However, when this metal was mixed with Fe and Zn, the inhibition of SD activity was lower (67.82%) than with Cd alone, showing an antagonistic effect and AP increased its activity (73.26%). Mitochondria were structurally damaged by exposure to Cd alone. However, in the metal mixtures, the toxic effects may exert interactive effects eliciting a less structural damage in the mitochondria of epithelium cells of the digestive tract than when Cd is alone.