The blood-brain barrier (BBB) to endogenous albumin was studied in normal adult, newborn and aged mice by using quantitative immunocytochemical procedures. For control purposes, the non-BBB-type of microvasculature from skeletal muscle (diaphragm) was also examined. Thin sections of tissue samples embedded at low temperature in hydrophilic resin Lowicryl K4M were exposed to anti-mouse albumin antiserum followed by protein A-gold. Labelling density was recorded over four compartments: vascular lumen, endothelium, subendothelial (perivascular) space and adjacent neuropil. The labeling density of the vessel lumen was considered to represent 100% of circulating albumin. The main differences between the two types of microvasculature consisted in the appearance of one-fifth of the labeling density of circulating albumin in the perivascular space of the non-BBB-type and only about one one-hundredth in the perivascular space of the BBB-type microvessels. The results suggest that the barrier function of brain microvasculature to endogenous albumin in newborn and aged mice is not significantly different from that in normal, adult animals. However, applied morphometrical and quantitative analysis of the density of immunolabeling over the above-mentioned compartments makes it possible to detect even subtle deviations from normal values in both extreme-age groups.