The early effects of intracerebrally infused vascular endothelial growth factor (VEGF) on the blood–brain barrier (BBB) to endogenous albumin were studied using a quantitative immunocytochemical procedure. In addition, transmission electron microscopy was used to observe morphological changes induced in brain vasculature. A solution of VEGF in saline (40 ng/10 μl) was infused into the parieto-occipital cortex of mice, which were killed 10 min, 30 min, and 24 h afterwards. Untreated mice and mice that received infusion of saline only were used as controls. For immunocytochemical evaluation, ultrathin sections of immersion-fixed brain samples embedded in Lowicryl K4M were exposed to anti-albumin antiserum followed by protein A-gold. Simultaneously, other brain samples embedded in Spurr resin were used for ultrastructural examination. Morphometric and statistical analysis indicated that as soon as 10 min after infusion of VEGF, 33% of vascular profiles were leaking albumin, and this value increased at 30 min to 92%. This effect of VEGF appears to be of rather short duration because after 24 h, only 27% of vascular profiles showed signs of leakage. The results of ultrastructural observations indicate that VEGF (30 min post-infusion) induces several changes in microvascular segments located in the area of intracerebral infusion of VEGF. These changes consist of the appearance of interendothelial gaps; fragmentation of the endothelium with formation of segmental, fenestrae-like narrowings; degenerative changes of the vascular basement membrane; and the appearance of fibrin gel in the vessel lumen. At 24 h post-infusion, solitary diaphragmed fenestrae appeared in attenuated segments of the endothelium in a few microvascular profiles. These changes, which are interpreted to be preparatory steps for angiogenesis, affect the structural integrity of the vascular segments, leading to extravasation of blood plasma proteins, including albumin. © 1998 Chapman and Hall