Abstract: A fluidized catalytic cracking process using a high efficiency regenerator comprising a coke combustor, a dilute phase transport riser, and a second fluidized bed with catalyst recirculation to the coke combustor, is operated to reduce NO.sub.x emissions in the regenerator flue gas. The amount of catalyst recirculation from the second fluidized bed to the coke combustor or combustion air addition or preferably both are adjusted based on continuous or periodic measurement of a process parameter of the FCC regenerator which directly or indirectly measures the NO.sub.x content of regenerator flue gas. Operation with restricted air or catalyst recirculation degrades coke combustor operation, shifts some regeneration to downstream portions of the regenerator, and reduces NO.sub.x emissions.