Studies have been made for the application of liquid poisons in lieu of moving control rods for shim control of core reactivity. Liquid control is achieved by: (1) injection of neutron-absorbing poison into the system from a poison supply tank if lower core reactivity is desired; (2) removal of a certain percentage of neutron-absorbing poison from the system by ion exchange if higher core reactivity is required; (3) no poison is added to or subtracted from the system if no reactivity change is desired. There is a wide choice of absorbers which could absorb neutrons in the thermal and epithermal ranges because most of the nitrates of these absorbers are soluble. Nitrate or other salts of cadmium, europium, or gadolinium are suggested for absorbing thermal neutrons, while silver, indium, or hafnium salts are used for the removal of resonance neutrons. A mixed solution containing one or more of these salts in any desired ratio can be prepared according to the need of a particular reactor. Boric acid can also be used. The principal advantages of using chemical poisons are: (a) lower capital cost; (b) simpler maintenance; (c) ready control of large reactivities; and (d) elimination of rod hot-spot factors. The liquid control system under consideration was studied for its applicability to nuclear rocket reactor control, although it might also be feasible for the control of ordinary power reactors with certain modifications.