A fluid delivery device for delivering a substance to a patient by way of infusion delivers the preparation at a rate o flow which varies in steps from a substantially constant higher rate, to a stepped-down substantially constant lower rate or rates. The delivery device includes one or more reservoirs (A ,B, N) and one ore more Belleville springs for applying generally constant pressures to the substance contained in the reservoirs. Each reservoir will have a different constant pressure applied in a mid-range of operation. The reservoirs can be interconnected (10) to each other and to an infusion device (13) in a number of arrangements, including various manifolds and flow restrictors (11), such that the rate of flow is controlled in steps in accordance with the pressures applied by the springs of the plurality of reservoirs.
