Pollution by metals may adversely affect organisms through the generation of reactive oxygen species (ROS). In this study, we examined the sublethal effects of two metals, copper and cadmium, on horseshoe crab (Limulus polyphemus) embryos. Exposure to copper or cadmium at concentrations of 0.01–10 mg/L for periods of 4, 8, 16 and 24 h had minimal effect on embryo survival except at 100 mg/L Cu. However, metal-exposed embryos took significantly longer to hatch into first instar (“trilobite”) larvae than seawater controls. Levels of superoxide dismutase (SOD), believed to be important in the response to oxidative stress, were determined by Western blotting. Both the Cu/Zn and Mn cofactor forms of SOD tended to be somewhat elevated in metal-exposed embryos, but the increases were neither dose nor time-dependent. Likewise, SOD enzymatic activity showed no significant differences comparing embryos exposed to metals with seawater controls. We conclude that the protective role of SOD’s against ROS produced in response to metal exposure appears to be limited in horseshoe crab embryos, at least under our experimental conditions.