Abstract: An apparatus for ambulatory intravenous delivery of substances to a recipient comprising a container and delivery means for pressurized delivery of the substance. The container communicates with a source of pressurized gas to provide the pressurizing force and is uniquely attachable to the recipient so as to provide portability to the system. The flow rate is controlled by feedback electronics connected to a flow rate meter and a throttle valve. The flow rate meter has a heating resistor in the flow path of the substance and a thermistor to detect a temperature rise in the substance downstream from the resistor. A constriction in the flow tube is disposed between the resistor and thermistor to cause a flat wavefront of temperature increase. The throttle valve is a pin disposed partially within a tube to form an annular gap. The flow rate is adjusted by changing the distance the pin extends into the tube and thereby change the length of the annular gap.