In order to determine whether any structural changes occur in the blood-brain barrier during development which can be related to changes in barrier function over the same period, morphometric methods were used on pieces of visual cortex taken from rats between 16 days gestation and adult and processed for light and electron microscopy. Capillary volume fraction, numbers per unit area, and surface density increased in two phases, before birth and between 10 and 20 days after birth, with no subsequent change after 20 days. Brain parenchymal mitochondria, assumed to be a measure of brain oxidative activity, did not change before birth but increased gradually and continuously after birth from 1.7% to 5.7% in adults. The capillary endothelial mitochondrial volume fraction as a percentage of the endothelial cell cytoplasm, previously thought to be important for ion transport, did not change with age (mean = 5.4%), although there was evidence that mitochondria either divide or change in shape up to 30 days after birth. The endothelial cell thickness decreased with age from 0.6 to 0.2 microns, probably through cell elongation during vascular growth. By combining the data on endothelial mitochondria and vascularity, it was shown that the total endothelial mitochondrial volume per unit length of capillary decreased with age, whereas per unit volume of tissue it increased.