Objective: These experiments were conducted to investigate the optimal expose length of propidium iodide (PI) and bisbenzimide on differential staining of mouse blastocysts. Materials and methods: A total 964 blastocysts (early${sim}$hatched) was exposed to PI (n=831) (group I: $leq$ 10; II: $11{sim}15$; III: $16{sim}20$; IV: $geq$21 sec) and bisbenzimide (n=133) (group A: $leq$1; B: $1{sim}$3; C: $geq$ 4 hr) in several periods for differential staining. Statistical analysis was performed using t-test with SigmaPlot-2001. P-values < 0.05 were accepted as statistically significant. Results: In case of PI exposure, differential staining rates were significantly higher (p<0.05) in group I (89.8%) than in any others (group II: 77.6%; III: 29.6%; IV: 22.2%) and higher (p<0.05) in group II than in group III and IV. In case of bisbenzimide exposure, differential staining rates were not statistically differences in three groups (group A: 97.4%; B: 87.8%; C: 93.3%). Conclusion: The differential staining rates of mouse blastocysts are not affected by the exposure length of bisbenzimide. However, blastocysts were exposed to PI with period of shorter than 15 sec show best outcomes of differential staining rates.