Diclofenac is embryotoxic in animals but the mechanism of its embryotoxicity is not fully understood. We postulated that diclofenac-induced embryotoxicity might be related to free oxygen radical damage. Rat embryos were explanted at 9.5 days gestation and cultured in medium containing various concentrations of diclofenac. The direct effect of diclofenac on embryonic free oxygen radical content was evaluated by measuring the 8-isoprostaglandin F2alpha level. We found that embryos exposed to high concentrations of diclofenac (7.5 and 15.0 micro g/ml) had significantly higher levels of 8-isoprostaglandin F2alpha (2.85 and 3.50 pg/mm embryonic crown-rump length, respectively) than the control group (1.73 pg/mm, P<0.05), while the 8-isoprostaglandin F2alpha level in embryos exposed to low concentration of diclofenac (1.5 micro g/ml) was 2.54 pg/mm, which was not significantly different from the control group. Median embryo morphologic score was also lower in the high concentration group compared to the control group (39.0 versus 44.0, P<0.05). These results suggest that diclofenac-induced embryopathy may be related to free oxygen radical damage.