A short exposure to 5 or 10 micrograms/ml cadmium chloride for 24 hours disturbed the in vitro development of four-cell and morula-stage embryos of F1 (C57 female X A2G male) mice. Morulae appeared to be less sensitive to cadmium than four-cell embryos. However, the in vitro development of four-cell embryos through compaction to morulae was not affected, though most treated embryos degenerated and decompacted later. It was proposed that cadmium toxicity may not be acting through contact effects on the cell surface and cytoskeleton. It probably interferes with the general energy metabolism of the cells. Although 1 microgram/ml cadmium did not disturb the subsequent in vitro development beyond the implantation stage, a reduced capacity of implantation in vivo was observed after surgical transfer to recipients. In spite of the effects of cadmium salts on the maternal side, the present results suggest that a direct embryotoxic and teratogenic activity of cadmium cannot be excluded.