The efficiencies of airway heat and moisture exchanging filters in reducing respiratory water losses and increasing airway temperatures during general anesthesia were studied in five tracheally intubated patients given isoflurane, nitrous oxide, and oxygen anesthesia during controlled ventilation. Filters (Humid-Vent Filter, Humid-Vent 1, Pall Conserve, Siemens 150, and ThermoVent 600) were placed between the Y-piece of the anesthesia circle system and the endotracheal tube for 40 min each. Airway temperature, esophageal temperature, and water loss (determined by weighing expired water collected in CaSO4) were measured every 10 min. All of the filters reached near-maximum efficiency in reducing water losses within 10 min. The Humid-Vent Filter and Siemens 150 filters were most efficient, the Pall Conserve and ThermoVent 600 less efficient. Airway temperature rapidly increased 2 degrees-8 degrees C during each trial. The more efficient the filter in conserving water, the greater the airway temperature. The respiratory heat conserved by these filters represents 5.5%-7.2% of the estimated total metabolic heat production during anesthesia in adults.