['Mechanisms of initiation, progression and rupture of cerebral aneurysms have not yet been fully understood despite its clinical significance. Matrix metalloproteinases (MMPs) are a family of proteinases which are involved in the remodeling of vascular walls. In the present study, we investigated the significance of MMPs in the progression of cerebral aneurysms.', 'Cerebral aneurysms were experimentally induced in 7-week-old male Sprague-Dawley rats. MMP-2 and MMP-9 expression was examined by immunohistochemistry and RT-PCR. Gelatinase activity in aneurysmal walls was assessed by in situ zymography. A selective inhibitor for MMP-2, -9 and -12, tolylsam, was used to examine the effect of inhibition of MMP-2 and MMP-9.', 'Macrophages infiltrated in arterial walls of experimentally induced rat cerebral aneurysms and expressed MMP-2 and -9. Macrophage infiltration and MMP expression was increased with the progression of aneurysms. Gelatinase activity attributable to MMP-2 and MMP-9 increased in arterial walls of rat cerebral aneurysms. Furthermore, tolylsam reduced the ratio of advanced aneurysms in our rat model.'] Do macrophage-derived matrix metalloproteinase-2 and -9 promote the progression of cerebral aneurysms in rats?