[en] Purpose: To investigate the impact of γ-irradiation on cyclooxygenase-2 (COX-2) expression and its enzymatic activity in PC-3 cells. Cell cycle redistribution, viability, and apoptosis were quantitated in control and irradiated cells with or without the COX-2 inhibitor NS-398. Methods and Materials: Western blot analysis was used to assess COX-2 protein expression. Prostaglandin (PGE₂) was measured after addition of arachidonic acid (AA) using a Monoclonal Immunoassay Kit. Cell cycle and apoptosis were assessed using flow cytometry. Results: We observed a dose-dependent increase in COX-2 of 37.0%, 79.7%, and 97.5% following irradiation with 5, 10, and 15 Gy, respectively. The PGE₂ level of irradiated cells was higher than in controls (1512 ± 157.5 vs. 973.7 ± 54.2 ρg PGE₂/mL; p < 0.005, n 4) while cells irradiated in the presence of NS-398 had reduced PGE₂ levels (218.8 ± 80.1 ρg PGE₂/mL; p < 0.005; n = 4). We found no differences in cell cycle distribution or apoptosis between cells irradiated in the presence or absence of NS-398. Conclusions: COX-2 protein is upregulated and enzymatically active after irradiation, resulting in elevated levels of PGE₂. This effect can be suppressed by NS-398, which has clinical implications for therapies combining COX-2 inhibitors with radiation therapy