Abstract: The invention relates to a liquid flow control apparatus that is particularly useful for adding a small amount of a first liquid per unit volume to a flow of a second liquid. One particular application is for medicators for adding a medicant to a water supply for farm animals. The invention allows the size of an orifice--which produces limited flow of water supply, and a pressure drop--to be changed in such a manner that flow of water need not be stopped nor need any parts of the apparatus be disassembled. Preferably a slide is mounted in a block to which flow pipe parts are connected, the slide having at least four differently sized orifices formed therein. O-ring seals disposed in contact with the slide in an interior cavity of the block, and surrounding connections of the pipe parts to the block, prevent leakage of flowing liquid. Also, a detent is provided for detenting the slide into each position in which an orifice is in alignment with the flow pipe parts.