Motivational analysis of a representative group of individuals who volunteer to participate in centrifuge acceleration research at the USAF School of Aerospace Medicine was performed. Monetary reward and curiosity were the main reasons for volunteering to participate in the research program. Fear and monetary reward not worth the risk were the main reasons for not volunteering to participate. The major reason for stopping participation in the acceleration research program was not because of volunteer subject-related reasons, but because of being asked to stop for medical reasons by the medical members of the research staff. This type of periodic motivational analysis serves a number of constructive purpose, including self review of human-use research practices and volunteer subject feedback which enhance methods for recruitment and retention of high quality volunteer subjects.