An aerosol generator (21) includes a flow passage (27) having an inlet (29), and an outlet (31); a heater (33) arranged relative to the flow passage for heating the flow passage, a source of material (37) to be volatilized in communication with the inlet of the flow passage; a valve (35) to open, and close communication between the source of material; the inlet of the flow passage; and a pressurization arrangement (39) 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 (41) for operating the heater, the valve; and a control device (43) for controlling supply of power from the source of power to the heater, and the valve. A metering device (463) in an inhaler (401) includes a pressurized source of medicated fluid (408), and a metering chamber (407) configured to deliver a predetermined volume of fluid to a heated flow passage (409) in the inhaler.