Peroxisome proliferator-activated receptors (PPAR) alpha and gamma are expressed in smooth muscle cells (SMC). This study was designed to compare the effects of PPARalpha and PPARgamma on SMC proliferation and migration, and to determine how they operate. Treatment of SMCs from porcine coronary artery revealed that mitogen-stimulated DNA synthesis was blocked by the PPARalpha ligand WY14,643 and 15d-PGJ2 (a putative PPARgamma agonist), but not by the PPARgamma agonist rosiglitazone or the PPARbeta/delta ligand GW501516. Inhibition of DNA synthesis by clofibrate and GW7647 confirmed that SMC proliferation is affected by PPARalpha. This conclusion was supported by the fact that WY14,643 also inhibited the proliferation of H4IIE hepatoma cells (expressing only PPARalpha) but not A10 SMCs (expressing only PPARgamma1). In contrast, the effective inhibition of all cell types with 15d-PGJ2 indicated this compound likely operates via a PPARgamma-independent mechanism. Interestingly, rosiglitazone did not inhibit DNA synthesis by either H4IIE or A10 cells, suggesting activation of PPARgamma does not influence cell proliferation. Phosphorylation of cdk2 and expression of PCNA were inhibited by WY14,643, but not rosiglitazone or 15d-PGJ2, indicating that PPARalpha prevents progression into S phase. Although rosiglitazone did not block SMC proliferation, it (like WY14,643) reduced neointimal hyperplasia in vitro. This observation can be rationalized by the fact that both WY14,643 and rosiglitazone inhibit SMC migration, likely through MMP9. Our study therefore shows that selective interference with mediators of cell cycle progression and cell migration via activation of PPARs may prevent growth-related vascular diseases such as restenosis and atherosclerosis.