A flow shut off or throttling valve is provided in a sprinkler nozzle housing to enable a nozzle to be changed without having to turn off a flow pressure source. The valve intersects a flow path through the nozzle housing and has an opening such that when the opening is aligned with the flow path, a flow stream can flow unobstructed through the flow path. The valve is movable between a fully open position in which the opening is aligned with the flow path and a closed position which blocks the flow stream from flowing to a nozzle disposed at an outlet passage of the flow path. The valve may be constructed to be either slidable or rotatable between the two positions, and is actuated by a gearing arrangement which is operable at the exterior of the nozzle housing. The external valve actuator may function as a physical barrier to retain the removable nozzle in the nozzle housing when the valve is open and to disengage the nozzle when the valve is closed.