The cerebromicrovasculature of immature mice ranging in age from 1-24 days after birth was studied by electron microscopy. A micromanipulator apparatus enabled us to facilitate direction of a needle cannula and injection into leg veins or hearts with horseradish peroxidase (HRP) or ferritin (anionic and cationic) tracers and subsequent perfusion fixatives. Numerous HRP-filled vesiculocanalicular transport structures appearing in endothelial cells (ECs) of brain microblood vessels (MBVs) were observed in time periods ranging from 1-14 days. In addition to HRP transport across the ECs by tubulovesicular profiles, some of these structures appeared to become connected to multivesicular bodies. Between 14 and 24 days after birth, limited HRP was transported across the ECs to the basement membrane in only a few short segments of subpial arterioles. The decoration pattern with cationized ferritin (CF) on the luminal surface of the ECs depends upon whether the surface was exposed to the ligand before or after fixation. Quenching of aldehyde groups in fixed brain tissue has critical importance for the decoration pattern of CF on the luminal plasmalemmal surface. The absence of CF labeling on the delimiting membranes of plasmalemmal vesicles and tubular structures suggests that these structures represent differentiated microdomains engaged in macromolecular transport in the ECs of the developing mouse brain.