Propylthiouracil (PTU), in addition to its antithyroid effect, is recently found to have a potent antiatherosclerotic effect. Because collagen accumulation is the major contributor to the growth of atherosclerotic lesions and the neointimal formation after arterial injury, the aim of this study is to investigate the impact of PTU on collagen regulation. In the rat carotid injury model, PTU administration reversed the up-regulation of collagen in the neointima induced by balloon injury. In vitro, vascular smooth muscle cells (VSMCs), the main origin of arterial collagen, were treated with PTU. Propylthiouracil caused a concentration-dependent decrease in collagen I and III steady-state protein and mRNA levels, as determined by immuno-cytochemistry, Western, and/or Northern blot analyses. Transient transfection experiments using rat type I collagen promoter construct showed that PTU failed to affect collagen gene transcription in VSMCs. Actinomycin D studies demonstrated that the half-life of collagens mRNA decreased with PTU treatment, suggesting that PTU down-regulates collagen expression predominantly at the post-transcriptional level. Taken together, these data suggest that PTU inhibits VSMC collagen production via destabilization of collagen mRNA that contributes to its beneficial effect on atherogenesis and neointimal formation after arterial injury. However, whether the destabilization of collagen may induce plaque rupture in PTU-treated arteries merits further investigation.