This study investigates the impact of transient engine operation on the emissions formed during a tip-in procedure. A medium-duty production V-8 diesel engine is used to conduct experiments in which the rate of pedal position change is varied. Highly-dynamic emissions instrumentation is implemented to provide real-time measurement of NOX and particulate. Engine subsystems are analyzed to understand their role in emissions formation. As the rate of pedal position change increases, the emissions of NOX and particulates are affected dramatically. An instantaneous load increase was found to produce peak NOX values 1.8 times higher and peak particulate concentrations an order of magnitude above levels corresponding to a fivesecond ramp-up. The results provide insight into relationship between driver aggressiveness and diesel emissions applicable to development of drive-by-wire systems. In addition, they provide direct guidance for devising low-emission strategies for hybrid vehicles. Hybrids offer more flexibility in controlling the engine, but optimizing their supervisory control for fuel economy can lead to frequent and sharp load increases that can contribute greatly to higher overall emissions levels. The characterization of transient effects on emissions can be applied towards shaping of tip-in functions for optimal economy-emissions tradeoff in hybrids.