AIM: To observe the effect of hemin on the expression of heme oxygenase-1(HO-1) in rat vascular smooth muscle cells(VSMCs). METHODS: Wistar rat aortic VSMCs were cultured in vitro and induced to proliferate by angiotensinⅡ(AngⅡ),hemin(a substrate and inducer of HO-1) and zinc protoporphyrin-Ⅸ(ZnPP,an inhibitor of HO-1)were added to induce and inhibit the expression of HO-1,respectively.The expression of HO-1 mRNA and protein were detected using reverse transcription polymerase chain reaction (RT-PCR)and Western blot,respectively. The relative amount of CO released into the media was quantitated as carbon monoxide hemoglobin(COHb)by enzyme-linked immunosorbent assay(ELISA),the proliferation of VSMCs was detected by MTT. RESULTS: The expression of HO-1 mRNA and protein in VSMCs and the amount of COHb in the media were increased significantly by hemin.Meanwhile the proliferation of VSMCs was suppressed markedly. CONCLUSION: The change of HO-1 mRNA and protein expression is the molecular base of the antiproliferation of endogenous CO .The HO-CO system might play a significant role in the development of cardiovascular diseases characterized by proliferation of VSMCs.