Aim To investigate the effects of interferon γ(IFN γ) on rat vascular smooth muscle cell(VSMC) proliferation and its expression of proliferating cell nuclear antigen(PCNA). Methods The changes of tritium labelled thymidine ( 3 H TdR) incorporation into rat VSMC and PCNA expression (Western blotting) were observed after 24 h stimulation with IFN γ. Results IFN γ(500 ku/L) markedly suppressed serum stimulated 3 H TdR incorporation into VSMC from 7 865±1 054 cpm/well in the serum group to 4 914±1 078 cpm/well in the co cultured group with serum and IFN γ(p<0.01), and decreased PCNA expression from 167.62±8.03 in serum group to 120.65±10.2(Western blotting, absorbance) in the co cultured group(p<0.01). Conclusions The results suggested that IFN γ inhibit PCNA expression and DNA synthesis of rat VSMC, and suppress VSMC proliferation.