For more than a decade, the need for measuring exhaust emissions on the basis of weight of emissions per mile has been known. Because of the complexity involved in mass emission measurements, the original procedures and standards were expressed in terms of concentration. Since adoption of these early concentration standards, there has been continued effort toward development of mass emission measurement techniques. One such procedure has been proposed by the National Air Pollution Control Administration which employs a constant volume blower and yields a concentration measure proportional to the mass flow. This equipment satisfies many of the objections to the previous mass measurement approaches, and is being considered for application in certifying 1972 model passenger cars. This procedure will measure higher mass emissions on both controlled and uncontrolled cars than those calculated using the 1970 procedure.General Motors has been working with the constant volume sampler for the past year and a half. While this approach is competitive with any other known mass emission measurement technique, its meaningful use depends on many factors that must be closely controlled since they affect the emission results. Before the procedure can be uniformly practiced, many more details of its use must be specified, and presently known sources of error must be resolved. Since all of these factors affect the weight of emissions that will be measured, rational standards cannot be adopted until the procedure is better defined. The various design requirements for the equipment are discussed along with the known limitations.