The effects of prostacyclin and its stable analogues on endothelin-induced DNA synthesis were investigated in cultured vascular smooth muscle cells. Aortic smooth muscle cells were obtained from male Wistar rats. In order to eliminate endothelial cells, the cells were cloned. DNA synthesis in intact cells was estimated by [3H]thymidine incorporation. Prostacyclin and its stable analogues (TEI-7165, TEI-9090, TEI-1324, TEI-3356, and TEI-9063) inhibited the endothelin-induced DNA synthesis with an IC50 of 1-30 nM. These results indicate that prostacyclin and its stable analogues are possibly effective in preventing the proliferation of vascular smooth muscle cells under some pathological situations, including atherosclerosis.