The fenestration of the vertebral and basilar arteries was investigated macroscopically in 370 human cases. Altogether 43 fenestrations were found in 32 (8.6%) out of 370 cases. The fenestrations could be divided in three groups according to the position where the fenestration appeared. Group I was the fenestration observed along the vertebral artery (Fig. 1), and 7 such fenestrations were found in 6 (1.6%) out of 370 cases; 3 fenestrations among them were located on the right side, 2 were located on the left side and 1 was found on the bilateral vertebral arteries. In 4 out of 7 fenestrations in this group, the hypoglossal nerve was found passing through the fenestration. Group II was the fenestration observed along the basilar artery (Fig. 2), and 26 fenestrations of this type were found in 20 (5.4%) out of 370 cases: 23 fenestrations (88.5%) were located at the caudal half, 3 (11.5%) were located on the rostral half of the basilar artery (Fig. 5). A close relation was indicated between the side on which the fenestration appeared in the basilar artery and the largeness in caliber of the vertebral artery of either side. Group III was the fenestration observed at the lateral area of the vertebral and basilar arteries (Fig. 3), and 10 fenestrations of this type were found in 9 (2.4%) out of 370 cases; all the fenestrations were located on the right side. In this group, the hypoglossal nerve was observed passing through the fenestratin in 4 out of the 10 fenestrations.(ABSTRACT TRUNCATED AT 250 WORDS)