The objective of this study was to evaluate the effects of the exposure to cadmium on the antioxidant responses in the polychaeta Laeonereis acuta. The worms were submitted to 0, 5, and 100 microg of Cd/L during a period of test of 7 days. Cadmium was significantly (p<0.05) accumulated in L. acuta in both concentrations assayed, but the concentration of reactive oxygen and nitrogen species (RONS) increased (p<0.05) only in the group submitted to the highest concentration of cadmium (100 microg/L). At this concentration, a decrease in the activity of the superoxide dismutase and an increase of glutathione-S-transferase activity (p<0.05) was observed. The levels of both lipid peroxides and the activities of catalase and glutathione peroxidase were not affected (p>0.05) by the exposition to cadmium. Thus, cadmium can augment RONS levels and can interfere with the antioxidant defense system of the polychaete L. acuta, although cadmium does not directly induce oxidative stress unlike copper and iron.