This paper empirically tests several hypotheses offered by political scientists and economists to explain membership in political interest groups by examining the aggregate membership of the National Rifle Association (NRA) from 1921 to 1984. Intervention and transfer function ARIMA analysis is used to model membership on the basis of (1) qualitative changes in the level of material selective benefits offered prospective members, and (2) fluctuations in the number of people being discharged from the military. The NRA has an extensive history of institutional sponsorship. NRA membership increased dramatically with the onset of new selective material benefits in 1979 but did not decline when a reduction in selective material benefits occurred in 1968. Demobilization at the end of World War II had a significant impact on NRA membership and a positive relationship exists between NRA membership and the level of annual discharges from the military.