The results of erroneous filling of agent-specific anesthetic vaporizers were studied. The fraction of gas flow through the vaporizer was calculated for three vaporizers set to deliver essentially equipotent final concentrations: halothane, 1% (1.25 MAC); enflurane, 2% (1.19 MAC); and isoflurane, 1.5% (1.30 MAC). These fractional flows, at 22°C, were 0.0188 for 1% halothane, 0.0615 for 2% enflurane, and 0.0295 for 1.5% isoflurane. Concentrations were calculated for cases of total filling of a vaporizer with one of the other two agents. In terms of potency of delivered agent, fourfold underdoses or overdoses could result from such errors. Refilling a 25% full vaporizer with the wrong agent then was considered. In order to calculate the concentrations of each agent that would be delivered in such a case, vapor pressures of each were determined in mixtures of two agents. Enflurane and isoflurane could not be separated satisfactorily by gas chromatography. Halothane, when mixed with enflurane or isoflurane, enchanced vaporization of each agent, as well as being somewhat more easily vaporized itself. Halothane, enflurane, and isoflurane do not form ideal solutions when mixed and the resultant vapor concentrations of each of two agents when mixed may be far from those predicted by an assumption of ideality.