In adult rats when plasma osmolality increases, water flows across the blood-brain barrier down its concentration gradient from brain to plasma and brain volume deceases. The brain responds to this stress by gaining osmotically active solutes, which limit water loss. This phenomenon is termed brain volume (water) regulation. We tested the hypothesis that brain volume regulation is more effective in young lambs and adult sheep than in fetuses, premature and newborn lambs. Brain water responses to acute hyperosmolality were measured in the cerebral cortex, cerebellum and medulla of fetuses at 60% and 90% of gestation, premature ventilated lambs at 90% of gestation, newborn lambs, young lambs at 20-30 days of age and adult sheep. After exposure of the sheep to increases in systemic osmolality with mannitol plus sodium, brain water content and electrolytes were quantified by standard techniques. The ideal osmometer is a system in which impermeable solutes do not enter or leave in response to an osmotic stress. There were significant differences from that of an ideal osmometer in: the cerebral cortex of fetuses at 90% of gestation, cerebral cortex and cerebellum of newborn lambs, and cerebral cortex, cerebellum and medulla of young lambs and adult sheep; but there were no differences in: the brain regions of fetuses at 60% of gestation and premature lambs, cerebellum and medulla of fetuses at 90% of gestation and medulla of newborn lambs. We conclude that: (1) brain water loss is maximal and brain volume regulation impaired in most brain regions of fetuses at 60% and 90% of gestation and premature lambs, (2) brain volume regulation develops first in the cerebral cortex of the fetuses at 90% of gestation and in the cerebral cortex and cerebellum of newborn lambs, and then in the medulla of the lambs at 20-30 days of age (3) brain water loss is limited and volume regulation present in the brain regions of young lambs and adult sheep; and (4) the ability of the brain to exhibit volume regulation develops in a region and age-related fashion.