OBJECTIVE: To study the proliferation change of tunica intima and smooth muscle in artery after hydrolic dilation for potential clinical use. METHODS: Sixten adult New Zealand rabbits were randomly divided into 4 groups, named group A, B, C and D. Right carotid arteries of rabbits of those 4 groups were dilated by hydrolic dilation with different pressures with 0 kPa, 40 kPa, 80 kPa, and 120 kPa respectively. The arterial calibers, thickness of tunica intima and smooth muscle were analyzed by automatic medical photograph analyzer immediately, 1 week and 2 weeks later respectively. RESULTS: The arterial calibers in the experimental group were larger than those in control group after immediate hydrolic dilation and 1 week later (P lt; 0.01). At 2 weeks, the arterial calibers in group B and D has no significant difference compared to group A (P gt; 0.05), and those in group C were larger than that of group A (P lt; 0.01). There were no significant difference in thickness of tunica intima and smooth muscle between the experimental group and control group (P gt; 0.05) after immediate hydrolic dilation. At 1 and 2 weeks after dilation, there were no significant difference between group A and group B (Pgt; 0.05), and those in group C and D were all larger than those in group A (P lt; 0.01). No obvious proliferation of tunica intima were observed in group B at 2 weeks after hydrolic dialation, but the proliferation of tunica intima could be observed in group C and D, especially in group D. CONCLUSION: Caliber of artery can be expanded by hydrolic dilation with higher pressure, but the proliferation of tunica intima and smooth muscle may be occurred in hydrolic dilation with higher pressure over 80 kPa, therefore it is safe to use hydrolic dilation with pressure no more than 40 kPa.