The microvessels of the telencephalons of Beagle puppies between newborn and 72 hours of age were investigated ultrastructurally at 24-hour intervals. The morphometry of the microvasculature from the germinal matrix was compared with that of the microvasculature from the borderzone cerebral cortex. The endothelial cell walls of the microvessels from these two sites were similar during the study period, but the lumina of the matrical microvessels were significantly larger than the lumina of the control cortical microvessels. The proportion of matrical microvessels with large lumina undergoes progressive attrition with time. The values (areas) for the lumina of the matrical microvessels, and their distribution, come to resemble those of the cortex. The morphometry of the cortical microvasculature is comparatively static. The data suggest that there is an active modification in the microvasculature of the germinal matrix of the Beagle puppy in the immediate postnatal period.