Cadmium chloride, at concentrations of 0.5 or 1 microgram/ml medium, did not affect the trophoblastic invasiveness of mouse embryos treated for 24 hours at 4-cell and morula stages. At higher concentrations of 5 or 10 micrograms/ml medium, most treated embryos in vitro underwent degeneration while a few survivors formed trophoblastic outgrowths with variable areas. Cadmium chloride, at a low concentration of 0.5 microgram/ml medium presented continuously to blastocysts after attachment in vitro, has significantly retarded the trophoblastic outgrowth areas and reduced the number of trophoblastic giant-cell nuclei, though the spreading blastocysts appeared morphologically normal. At higher concentrations of 1 or 5 micrograms/ml medium, cytoplasmic disintegration and detachment of trophoblasts were observed. It is suggested that cadmium may interfere with the cell division and/or the transformation of trophectoderm cells into giant cells, resulting in the retardation of the trophoblastic outgrowths.