This disclosure describes a fully automatic, compact and portable operational device for repetitious intravenous (parenteral) infusion of fluids such as blood or blood derivatives, plasma, platelets, serum, dextrose, Ringer's solution and saline contained in pliant bags, at flow rates that not only are normally encountered but also at massive flow rates that are rather often called for but for which no such automatic device has existed. This is accomplished in a manner that frees the operator completely from attending to the device except for changing over to additional fluid bags for successive administration of fluids in such pliant bags and without loss of time. The fast response of the device is due to the special design that insures fast response both in the pressure application and retraction parts of the cycle. The device produces automatically any desired constant pressure over a wide range of pressures onto the pliant bags, over the whole compression phase of the pressure cycle. The pressure is maintained at that constant value both in time and over the entire surface of the pliant bags of various sizes.