Under conditions of temperature and pressure commonly found in operating rooms, the anesthetics enflurane, halothane, and isoflurane act as nearly ideal gasses even at concentrations producing a saturated vapor phase. The deviations from ideality are 3.44 +/- 0.44% (SEM) for enflurane, 0.94 +/- 0.30% for halothane, and 2.96 +/- 0.29% for isoflurane. That is, 0.9 to 3.4% more anesthetic is vaporized than would be predicted from the vapor pressure of these agents.