Pre-implantation mouse embryos at the morula or early blastocyst stage were stained with acridine orange (AO) or ethidium bromide (EB) in order to assess their viability. There was a significant difference ( p < 0·05) between the mortality rate of embryos stained with the two concentrations of AO tested (0·125 mg/ml and 0·250 mg/ml) and the unstained control. There was also a significant difference ( p < 0·05) between embryos exposed to the dye for 2 min and the unstained control (40·7% vs 16%). Embryos stained with AO showed a light-to bright-green fluorescence with pronounced red-tinged areas within the cells. It was concluded that AO would not be suitable for assessing the viability of mouse embryos because it produced a similar coloured fluorescence in both dead and live cells and was toxic. Neither of the two concentrations of EB (20 μg/ml or 40 μg/ml) nor the two exposure times (3 min or 5 min) tested significantly increased the mortality rate of mouse embryos over that for the unstained control. EB produced green fluorescence in live cells and red fluorescence in dead cells of the stained embryos. EB does not appear to be toxic to pre-implantation mouse embryos, thus it may be a potentially useful tool for assessing their viability.