The anesthetic partial pressure causing a loss of righting reflex in mice (ED50) was determined for isoflurane at ambient total pressures ranging from 1 to 100 atmospheres absolute (ATA). The higher pressures, produced by adding helium, decreased isoflurane's apparent anesthetic effect: 48 percent more was needed at 100 ATA. This is consistent with previous work with N2O, where a 46 percent increase was found. The ED50 for isoflurane at 1 ATA is 0.0057 ATA, only 29 percent of that predicted by the Meyer-Overton relationship for mice.