Mice were exchange transfused with a fluorocarbon emulsion (FC-80). Their hemotocrits were reduced to levels of 10-15. They were examined within 1 hour of completion of normal or slightly accelerated in the majority of the mice. Acceleration may have been due to reduced "blood" viscosity. All mice with normal or reduced transit times displayed reversible increases in brain NADH levels when they were exposed to room air or 100% N-2 for brief periods, rather than to the atmosphere of 100%O-2 used to support life in these mice. A small group of mice with prolonged fluorescein transit tended to display NADH levels that failed to rise when O-2 was reduced in the inspired air. In this small subgroup, microcirculatory failure had apparently produced anoxia and maximal NADH levels even before the supply of inhaled O-2 was reduced. Many FC-80 mice displayed reversible decreases in brain pO-2 levels when inspired O-2 was reduced. Pentylenetetrazol activated the EEG of FC-80 mice. The EEG activity was reversibly reduced when the animals were briefly exposed to room air or N-2. These data suggest that fluorocarbon emulsions can support brain microcirculation, oxygenation, and electrical activity in vivo, a result which is consonant with the observations of others who have performed in vitro experiments, or observed long-term survival.