The aim of this experiment was to examine the regulation of p38 mitogen-activated protein (MAP) kinase by platelet-derived growth factor (PDGF)-BB and its biological effects on rat cultured vascular smooth muscle cells (VSMCs). VSMCs were obtained from aortae of male Wistar rats by the media explant technique. After being stimulated by PDGF-BB with or without the p38 MAP kinase-specific inhibitor, SB-203580, the cells were solubilized, and the levels of phosphorylated p38 MAP kinase were examined by immunoblot analysis. The amounts of DNA synthesis and content were measured by using [3H]-thymidine and Hoechst-33258 dye, respectively. The detection of apoptotic cells was evaluated by the TUNEL method. PDGF-BB could phosphorylate p38 MAP kinase dose-dependently, and the phosphorylation was specifically inhibited by SB-203580 in a dose-dependent manner. However, PDGF-BB did not affect the protein level of p38 MAP kinase. Both [3H]-thymidine incorporation and total cellular DNA content were increased by PDGF-BB, and these elevations were prevented by SB-203580. In contrast, PDGF-BB-stimulated VSMCs did not show apoptotic change in spite of the presence or absence of SB-203580. These results established that PDGF-BB activated p38 MAP kinase and subsequently regulated cell growth in VSMCs, providing a molecular mechanism by which p38 MAP kinase can cause the development of cardiovascular diseases, including atherosclerosis.