Water humidity and temperature were measured in the proximal end of the inspiratory limb in anaesthetic circuits used to anaesthetize three groups of adult patietnts using various fresh gas flows (FGF). Humidity increased as FGF's were lowered, with 98% water humidification achieved when FGF's of less than 0.51/min were administered. Temperature at the same site changed about 1-2.5 degrees C in inverse proportion to the volume of FGF. This advantage of closed system and low flow anaesthesia is a further reason for their wide-spread utilization.