A historical review of our knowledge of the subarachnoid space dates from the ancients through the modern electron microscope era. Conflicting observation resulted from various methods of tissue preservation and species variability. A comparative submicroscopic study shows striking similarities in the ultrastructure and distribution of the subarachnoid space in mice, cats, monkeys and man. Development of the pia-arachnoid membranes in the mouse occurs in four stages: the first follows closure of the neural tube and is a period of initial vascularization of the developing telencephalon; the second is a period of delineation during which the limits of the subarachnoid space are defined; the third is a period of ensheathment of pia-arachnoidal blood vessels; and the fourth includes addition of smooth muscle to larger vessels, the appearance of macrophages in the subarachnoid space, and a general increase in extracellular collagenous and elastic fibers. The subarachnoid space over the telencephalic surface in the 10-day fetus exists prior to the secretion of cerebrospinal fluid as the typically large extracellular space of mesenchyme. By the 13th fetal day cerebrospinal fluid begins to seep into and replace the ground substance of the mesenchyme. The mesenchymal extracellular compartment is reduced peripherally, resulting in a compacted pia-arachnoidal tissue which limits the peripheral extent of the subarachnoid space. By the 21st postnatal day a subarachnoid space typical of the adult animal has been established. The developmental sequence occurring in the tissues surrounding the central nervous system is important to our understanding of the pathogenesis of hydrocephalus and congenital anomalies.