The effects of heavy metals, such as Hg, Pb, Cu, Cd, As, Sn, Cr, Mn, Ni and Zn on the two developing sea urchin eggs. Temnopleurus and Hemicentrotus were investigated. The results obtained were as follows. 1. The development of fertilized eggs was inhibited when embryos were exposed to 10(-6) M of HgCl(2) and CuCl(2) and to 10(-4) M of CrCl(2) and NiCl(2) in Herbst's artificial seawater from fertilization until the formation of the blastula larva. 2. Chlorate and acetate of heavy metals were dissolved in filtered seawater. The morphological differences of embryo were not observed until the formation of the pluteus larva in the two solutions. Inhibitory effects were observed when embryos were exposed to seawater at concentrations of 10(-7) M of Hg and Cu, to 10(-6) M of Cd, and to 10(-5) M of Pb, Ni and Zn. At a concentration of 10(-4) M of Cd there were no visible effects until the appearance of swimming blastula larva, but larva development was strongly inhibited at time of bonification from pluteus larva, and there was no further later development. In 10(-6) M Hg, Cr, and Ni, the larvae were structural abnonal and shaped like Apollo. In 10(-7) M of Hg exogastrula embryos were produced. 3. Threshold concentration of complete inhibition on the early development of sea urchin embryos was at 10(-6) M of Hg, 10(-5) M of Cu, Cr, Ni, and Zn, 10(-3) M of Mn. In the case of Cd 10(-5) M, complete inhibition didn't occur until the pluteus larva stage. 4. Only additive effects were obtained on the developing sea urchin eggs when two heavy metals were combined in filtered seawater. A high frequency of morphological abnormalities were formed such as exogastrula and Apollo shaped pluteus especially in coupling of Hg and Cu or Cd and Zn. 5. There were no visible effects in filtered sewage from fertilization to the formation of the pluteus larva. When embryos were exposed to acetate salts of Hg, Cu, Cd, and Zn in filtered sewage, there were 10 fold inhibitory effects on development. But formation of skeleton in the pluteus larva was found in filtered sewage containing Cd. 6. Only additive effects were observed on sea urchin embryo when combined with Hg, Cu, Cd, and Zn in filterd sewage. But the abnormalities were decreased in all cases. Inhibition of bonification were reduced in mixture of Cd and filtered sewage.