We evaluated the individual and joint toxicity of zinc and cadmium during the early embryonic development of the zebrafish (2+ and Cd²⁺ 24 h post-fertilization (24 hpf). We monitored the number of embryonic coagulated eggs and the rhythm of the heart at 48 hpf and the number of embryonic coagulated eggs, hatching rate, and malfomation rate at 72 hpf. The effects of Zn²⁺ and Cd²⁺ differed among the stages of development. In general, Zn²⁺ was more toxic than Cd²⁺. The embryos were most sensitive to Zn²⁺ and Cd²⁺ toxicity at 72 hpf. Regardless of the ratio of Zn²⁺ and Cd²⁺ during joint exposure (11), the two chemicals tended to act synergistically. However, the joint toxicity was defendant on the endpoints and the ratio of each metal. For example, at equal concentrations of Zn²⁺ and Cd²⁺ (1the effect was synergistic at 48 hpf but antagonistic at 72 hpf. Furthermore, at a ratio of 2) the effect on egg coagulation was synergistic whereas the effect on the rate of hatching was antagonistic. Thus, our results suggest that joint toxicity is affected by both the concentration of each chemical and the exposure time.