Qing Nao Yi Zhi Fang (QNYZ), a Chinese medicine, has been developed as a drug for the prevention and treatment of vascular dementia. We examined the effects of QNYZ-treated serum and QNYZ on the proliferation of human arterial smooth muscle cells (SMCs) and endothelial cells (ECs). SMCs did not grow in a medium containing 10% QNYZ serum. QNYZ also completely inhibited the proliferation of SMCs at higher concentrations (500-1000 microg/ml), although no detectable inhibitory activity on the SMC proliferation was observed at lower concentrations (100-200 microg/ml). In contrast, QNYZ (250-500 microg/ml) significantly stimulated the proliferation of ECs and prostacyclin production in ECs. These results suggest that QNYZ may have suppressive effects on the development of intimal thickening in atherosclerosis and after arterial wall injury.