We have previously reported on the efficacy of a 20% fluorocarbon emulsion (Fluosol-DA, 20%) as an acellular O2carrier at an FIO2= 1.0. We are concerned, however, about the potential O2toxicity that may result from extended exposure to FIO2= 1.0. The O2content of the fluorocarbon phase is linearly related to both the FIO2and the fluorocarbon concentration (Fct). It should therefore be possible to maintain the same O2content by raising the Fct using a higher fluorocarbon concentration and lowering the FIO2. The purpose of this report is to assess the ability of a 35% fluorocarbon emulsion (Fluosol-DA, 35%) at an FIO2= 0.6 to support hemodynamics and O2transport.Five adult baboons were paralyzed, anesthetized, intubated, and mechanically ventilated at FIO2= 0.6. An isolvemic total exchange transfusion (E.T.). with Fluosol-DA, 35% was performed. Measurements were made at Hct's of 20, 10, 5, and < 2%. All animals survived the exchange. Total arterial O2content fell from 17.4 ± 0.7 to 3.3 ± 0.2. vol% (p< 0.01), and O2delivery decreased from 21.8 ± 2.2 to 5.1 ± 0.7 cc/min-kg (p< 0.01) during the exchange. There was no significant change in MAP, H.R., C.O., or VO2during the exchange transfusion. Fluosol-DA, 35% maintains normal hemodynamics and O2transport despite a marked fall in arterial O2content and total O2delivery. Fluosol-DA, 35% is thus an effective O2carrier at the safe FIO2of 0.6.