The effect of different concentrations of eicosapentaenoic acid (EPA), on rat pheochromocytoma PC12 cells were evaluated using cell viability, lactate dehydrogenase (LDH) activity, flow cytometric DNA analysis and electronic microscopy. A time- and dose-dependent decrease in the cell viability was observed in the cultures of PC12 cells, supplemented with EPA. The incubation with 200 microM EPA for 48 and 72 hr induced a decrease in the cell viability by 53.40 and 53.43%, respectively. Treatment of PC12 cells with EPA induced apoptosis in a concentration-dependent manner, as evidenced by flow cytometry analysis. The highest percentage of apoptotic cells accumulated to 30.32%, following treatment with 200 microM EPA. The LDH levels increased significantly on treatment with 100 and 200 microM EPA, by 144.4 and 197.3%, respectively, compared with the untreated control. In addition, the cell morphology change was also observed by electron microscopy. The results suggest that EPA mediates its effect on the PC12 cells, at least in part, via the induction by apoptosis.