Aim To explore the effects of homocysteine (Hcy) on the activity of matrix metalloproteinase-1 (MMP-1) and it's regulatory mechanism in cultured vascular smooth muscle cells. Methods Cultured rat vascular smooth muscle cells were treated with different concentration of Hcy in vitro for 48 h. The activity of MMP-1 was determined using the method of zymography. The expression of MMP-1 mRNA in vascular smooth muscle cells was observed by semi-quantitative reverse transcription polymerase chain reaction. Results Hcy significantly inhibited the activity of MMP-1 and down-regulated the MMP-1 mRNA expression in vascular smooth muscle cells in a dose-dependent manner. Conclusion These data suggested that Hcy can induce collagen accumulation by decreasing the secretion of MMP-1 and inhibiting the activity of MMP-1. It may be the key factor in the pathogenesis of atherosclerosis induced by Hcy.