The photo degradation kinetics of fenitrothion in various water media were examined under both direct and indirect photolysis with respect to degradation rate, half-life, and phototransformation kinetics of fenitrothion. The effect of fenitrothion and its photoproducts on steroid hormone biosynthesis was also investigated. The results indicate that the degradation rate of fenitrothion under indirect photolysis to which nitrate was added was faster than that of direct photolysis, in both pure and natural water. The fenitrothion half-lives under indirect photolysis were 2.67 and 4.58 h, while under direct photolysis the half-lives were 3.58 and 5.66 h for natural and pure water, respectively. The phototransformation kinetics of fenitrothion in pure water showed that the identified photoproducts, such as fenitrooxon and 3-methyl-4-nitrophenol, under both direct and indirect photolysis were almost the same. This is evidence that there is no specific degradation pathway with hydroxyl radicals under indirect photolysis in fenitrothion transformation. The 1 μM∼50 μM levels of fenitrothion and two of its photoproducts (fenitrooxon and 3-methyl-4-nitrophenol) altered the steroid hormones biosynthesis in bovine adrenal cultured cells, which suggests that both fenitrothion and its two photoproducts may be endocrine-disrupting compounds.