The effect of radiation on the steroidogenesis of rat ovary was investigated. For this purpose, female rats aged 7-8 weeks were exposed to a single dose of ${gamma}$ radiation(320 rad or 800 rad) by the cobalt-60. At fourth(day of the first estrus) and eighth(day of the second estrus) days after irradiation, the concentrations of serum steroid hormones were determined by radioimmunoassays (RIA). The correlation between survival rate(Y) and radiation dose(X) was Y=-0.06X+100(r=0.89, n=10). Lethal dose$(LD)_{50(30)}$ and $LD_{20(30)}$ were 833.33rad and 333.33rad respectively. The weights of body and ovary was decreased significantly by the $LD_{50(30)}$ irradiation during the 4 days, but both weights were recovered at day 8. The serum levels of 17a-hydroxyprogesterone(170HP) and estradiol($E_2$) in the irradiated rats were not generally different from those of control. However, the levels of testosterone(T) and 19-norterstosterone(19NT) in the irradiated rat at $LD_{50(30)}$ and $LD_{20(30)}$ radiation doses were decreased, while progesterone levels in serum of the irradiated rats were slightly increased. These results suggest that irradiation may inhibit the first step of steroidogenesis, especially the conversion of P to T, in the rat ovary.