Objective: To identify the cells responsible for mRNA production within the cerebral AVMs and their feeding arteries by in situ hybridization, and to study the cerebral AVMs by transmission electron microscopy (TEM). Methods: In situ hybridization was carried out with digoxigenin RNA probes to localize the cells responsible for MMP-2, MMP-9, TIMP-1 and TIMP-2 mRNA production in the histologic sections of the cerebral AVMs (n = 4) and their feeding arteries (n = 7). Results: The levels of MMP-9 mRNA were found rather high in all the lamellae of the cerebral AVMs, especially in the adventitia. But none of the MMP-9 mRNA was found in normal cerebral arteries. None of MMP-2, TIMP- 1, TIMP-2 mRNAs was found in either the cerebral AVMs or the normal cerebral arteries. In situ hybridization studies colocalized MMP-9 expression to monocyte, fibroblast, smooth muscle cell in the AVM lesions. Weigert studies showed the partial lesions of internal elastic lamina. TEM disclosed missing of almost all the endothelial cells. Conclusions: The missing of the endothelial cells was likely to be the foundation of cerebral AVM formation. The adventitia might also play an important role in the formation of cerebral AVM formation. The destruction of collagen in the adventitia might be due to the high level of MMP-9 mRNA.