Aim To determine the effects of insulin on prolifer-ation of vascular smooth muscle cells (VSMC) and ac-tivity of tissue type plasminogen activator (tPA) andplasminogen activator inhibJtor (PAI) .Methods The cell proliferation was detected by H-thymidine (³ H-TdR) incorporation. The activity oftPA and PAI were determined by spectrophotometricassay.Results Insulin increased the activity of PAI andstimulated cell proliferation. There was no activity oftPA in procine VSMC.Conclusion Hyperinsulinemia could be one of theimportant factors in causing atherosclerosis.