An aerosol generator includes a flow passage having an inlet and an outlet, a heater arranged relative to the flow passage for heating the flow passage, a source of material to be volatilized in communication with the inlet of the flow passage, a valve to open and close communication between the source of material and the inlet of the flow passage, and a pressurization arrangement for causing material in the source of material to be introduced into the flow passage when the valve is in an open position. The aerosol generator further includes a source of power for operating the heater and the valve, and a control device for controlling supply of power from the source of power to the heater and the valve. A metering device in an inhaler includes a pressurized source of medicated fluid and a metering chamber configured to deliver a predetermined volume of fluid to a heated flow passage in the inhaler. The metering chamber can be part of a rotary valve having a bore and a displacement member moveable within the bore from a first position where the fluid is loaded into the bore to a second position where the predetermined volume is ejected out of the bore. Another metering chamber has an elastic portion of a delivery passage in fluid communication with the pressurized source of liquid and the elastic portion of the delivery passage is deformed to eject the predetermined volume.