The disclosed method and apparatus provide for the addition of micro-spheroidal ("MS") Fluid Catalytic Cracking ("FCC") catalyst additives and/or bulk catalyst to an in situ circulating, active catalyst inventory (14) of a fluid catalytic cracking unit at times and rates precisely controlled to sustain an effective additive concentration. In a preferred version of the invention, each of a series of such apparatus (10) (comprising one such device (10) for each catalyst additive) is operated on a basic cycle time length during a portion of which the unit is engaged in active addition while in the remainder of the time, it is passive.