Preimplantation mouse embryos were used to investigate the toxic effect of nickel chloride and cadmium acetate on early embryo development in vitro. Embryos at the 2- and 4-8 cell stage were cultured in approximately 0.05 ml of mouse embryo culture medium (No. 16), overlaid with paraffin oil and incubated in a humidified atmosphere of 5% CO2 in air for 48 h. NiCl2 . 6H2O was added to the culture medium at concentrations of 10-1000 microM, Cd(CH3COO)2 . 2H2O at concentrations of 10-50 microM. Morphological criteria were used to check embryonic development. Ten micromolars of nickel chloride affected adversely the development of Day 2 embryos (2-cell stage), whereas 300 microM was needed to affect Day 3 embryos (8-cell stage). Toxic effect of cadmium acetate on Day 2 embryos was observed at a concentration of 10 microM.