Developmental changes in regional cerebral blood flow (CBF) responses to hemorrhagic hypotension during normoxia and normocapnia were determined using radioactively labeled microspheres to measure flow to the cortex, brainstern, cerebellum, white matter, caudate nucleus, and choroid plexus in three groups of chromically catheterized lambs: 90− to 100-d preterm fetal lambs (n= 9); 125− to 136-d near-term fetal lambs (n= 9); and new born lambs 5− to 35-d-old (n= 8). Heart rate, central venous pressure, and arterial blood pressure were monitored continously and arterial blood gas tensions, pH. Hb. and oxygen saturation together with regional CBF were measured periodically. Hemorrhagic hypotension produced a mean decrease in arterial blood pressure of 27 \pm 4, 23 \pm 2, and 41 \pm 4% in the three groups, respectively, whereas reinfusion of the lamb's blood resulted in a return to control blood pressure within 3% in all three groups. In the preterm fetal lamb, CBF decreased significantly in all regions during hypotension. In the nearterm fetal lamb, only blood flow to the cortex decreased significantly during hypotension. In the newborn lamb, only the choroid plexus demonstrated a significant decrease in blood flow during hypotension The lower limit of regional CBF autoregulation was identical to the resting mean arterial pressure in fetal life but significantly lower in newborn lambs. These experiments demonstrate for the first time that vulnerability to hypotension decreases with increasing maturity and that the brainstem, the phylogenetically oldest region of the brain, is the least vulnerable to the effects of hypotension at any age in the lamb model. (Pediatr Res28: 361–366, 1990) Abbreviations CBF, cerebral blood flow