AIM: To investigate the relationship between medial smooth muscle cell proliferation and intimal thickening.METHODS: Angiotensin converting enzyme inhibitor (temocapril-HCl, 10 mg?kg -1 ?d -1 ) was administered in various periods after rat arterial injury by balloon catheters. The labeling BrdU indexes of smooth muscle cell and intimal areas were observed. RESULTS: Positive medial smooth muscle cells for BrdU occurred at 24 hours after injury. The labeling indexes for BrdU showed biphasic change, the period of the first peak appeared within day 1-3, the peak value was 5 3% at 2 days. The period of the second peak appeared within day 3-7, the peak was 2 7% at 5 days. Administration of temocapril in the first proliferation phase significantly suppressed the labeling BrdU index(0 05?0 02)% vs (4 50?0 27)%, P0 05 ). CONCLUSION: The first proliferative phase of arterial medial smooth muscle cell after balloon injury is necessary to the intimal formation.