Abstract 1. 1. An ether-vapor regulator is described by means of which a permanent record of the anesthesia is obtained. Changes in circulation or respiration can be correlated with any changes in the ether-vapor concentration. 2. 2. The proportion of ether-vapor to air can easily and fairly accurately be regulated. 3. 3. A constant ether level can be maintained in the bottle, thus eliminating variations of ether-vapor concentration due to changes in liquid level. 4. 4. The ether bottle can be refilled without removing the mason jar cap. Thus in a prolonged experiment or series of experiments the jar can be anchored in place, obviating the necessity of readjusting the recording lever each time the bottle is refilled. 5. 5. The device can be used in studying effects of changes in vapor concentration and is suited to other volatile anesthetics such as chloroform. 6. 6. The device is designed for one-way air passage, thus eliminating rebreathing expired air from the dead space and in addition using less ether.