An bolus delivery system includes an implantable pump, a sensor for sensing an adverse patient condition, such as atrial fibrillation, and a catheter for delivery of a bolus of drug to a target area of a living body. The pump is provided with a bolus metering assembly which includes, in a preferred embodiment, an auxiliary bellows defining a bolus reservoir in addition to the main reservoir of the pump. The auxiliary bellows is selectively placed in fluid communication with the pressurized main supply of drug via an inlet valve to refill the bolus reservoir. An outlet valve is provided to permit egress of the bolus to the catheter from the bolus reservoir. A drive train including a stepper motor and a face cam selectively opens and closes the inlet and outlet valves to effect accumulation or metering and delivery of the bolus. The auxiliary bellows is preferably provided as a collapsible element resiliently biased to an expanded position and is collapsed under pressure in the main reservoir to expel the bolus.