Heavy rare earth elements (HREEs) were tested for adverse effects to early life stages of the sea urchin Sphaerechinus granularis. Embryos were exposed to analytically measured HREE concentrations ranging from 10(-7) to 10(-5) M. No significant developmental defect (DD) increases were observed in embryos exposed to 10(-7) M HREEs, whereas 10(-5) M HREEs resulted in significant DD increase up to 96% for HoCl3 versus 14% in controls. Embryos exposed to 10(-6) M HREEs showed the highest DD frequency in embryos exposed to 10(-6) M DyCl3 and HoCl3. Cytogenetic analysis of HREE-exposed embryos revealed a significant decrease in mitotic activity, with increased mitotic aberrations. When S. granularis sperm were exposed to HREEs, the offspring of sperm exposed to 10(-5) M GdCl3 and LuCl3 showed significant DD increases. The results warrant investigations on HREEs in other test systems, and on REE-containing complex mixtures.