Abstract: A gravel pack screen assembly adapted to produce fluids from two spaced apart zones of interest through gravel packing surrounding spaced apart gravel pack screens includes a sub interposed in the screen assembly between the two screens and having a reservoir for discharging a quantity of permeability reducing material into a layer of gravel packing intermediate the gravel packing surrounding each of the screens. The sub is operable to discharge the material into the gravel pack layer in response to movement of a washpipe or the like within the screen assembly to move an actuator member to cause a closure member to uncover ports in the sub in communication with the reservoir. The actuator member includes a transverse surface projecting into a central bore of the sub and operable to be engaged by deflectable collet fingers formed on a washpipe assembly or a similar member insertable within the gravel pack screen assembly to effect operation of the sub to discharge its permeability reducing material.