A pump assembly particularly useful as an infusor for parenteral administration of a small amount of a therapeutically active substance for a prolonged period of time at a substantially constant rate comprising a fluid imbibing pump mounted within a receptacle into which actuating fluid may be charged. The active agent is dispensed from the pump after the pump is exposed to the fluid. Means to vary or stop the pump by obdurating all or a portion of the exterior surface of the pump can also be provided. Variation in flow rates can also be obtained by changing the composition of the actuating fluid.