The cerebral cortices of young adult and immature Sprague-Dawley rats, 10, 14, and 21 days of age, were rapidly frozen in situ and substituted at –78.5°C with a 1% solution of osmium tetroxide in acetone. In random electron micrographs of each animal, each representing 42 μ2 of cerebral cortical molecular layer, the extracellular space was evaluated stereologically.

In all cases the extracellular space was found to be distributed as large extracellular lakes of variable dimension. Where no extracellular space was apparent adjacent cell processes were joined by 5-membered complexes, which appeared to represent fusion of immediately adjacent plasma membranes.

In 10-day old animals an extracellular space of 40.5% was found. This diminished progressively to 31.8% at 14 days and 26.3% at 21 days, which approached the adult value of 21.7%. These data were consonant with measurements of the maturing extracellular space based on uptake studies of tracers, water and electrolyte determinations and impedance measurements. They appear to represent accurately the changing distribution of extracellular water in rapidly maturing cerebral cortex.