OBJECTIVE: To ananlyze the direct effect of nitric oxide (NO), generated from sodium prusside (SNP) on the embryo developments in reproductive process. DESIGN: ova from mouse were treated to allow fertilization in in vitro culture. And the samples of fertilized ova were alloted into five alliqutos. Each alliquot was cultured in media treated with either concentration at 0 (n=92), 25microM (n=84), 50microM (n=80), 100microM (n=77), 500microM (n=54) of SNP. Main Outcome MEASURE: Rates of embryonal cell cleavages, viability and cell morphology were assessed during in vitro fertilization and culture. RESULTS: As analyse the cell cleavage at 24 hours after in vitro culture of fertilised egg in variuos NO concentration, all of egg cells of each alliquot were developed to 2~4 cell stage. But the alliquot of egg cells treated with 500microM, which were totally degenerated. And also all embryonal cells of each alliquot were developed to 8 cell stage and morula stage on culture continuosly. And the embryonal cells of each alliquot were analysed at 24 and 48 hours following the in vitro culture. The rates of cell fragmentation and fusion were 4.2+/-3.4% in control group which is not treated with NO, while experimental groups was high, as rated 23.4+/-6.2% in 25microM, 28.2+/-5.7% in 50microM and 32.1+/-6.4% in 100microM concentration of NO. Accordingly the rate of abnormal morphology of embryonal cell in control was lower significantly than that in each alliquot of experimental groups (p<0.05). And the degenerated rates of embryonal cells were 0% in control, 17.8+/-6.7% in 25microM, 23.6+/-4.7% in 50microM and 26.8+/-11.2% in 100microM at 8 cells and morula on culture of 48 and 72 hours. On the examination of embryonal cells developed to blastocyst through in vitro culture, the rates of degenerated cells were 16.8+/-7.2% in control, 37.5+/-6.2% in 25microM, 73.4+/-4.6% in 50microM, 100% in 100microM. CONCLUSION: This results suggeted that the No in any concentrations is harmful on embryos in view of morphology as well as viability of cell, and the toxicity of No on embryo is stronger at condition in higher concentration of NO.