The performances of six commercially available heat and moisture exchangers (HMEs) or "artificial noses," were evaluated in the laboratory. Each HME was studied with both a semiclosed anesthesia circuit (SCAC) and a nonrebreathing anesthesia system (NRS). Our results suggest that the Servohumidifier 150 and the Engström Edith are suitable for replacing nasopharyngeal humidification mechanisms bypassed during endotracheal ventilation in all patients because they provided at least 20 mg of water per liter of minute ventilation (mg/L) in an NRS and 28 mg/L in an SCAC during inspiration. While the other four units were suitable for small tidal volumes, they only marginally satisfied minimal suggested humidity levels in the presence of larger tidal volumes.