Increasing exercise behavior remains an important health initiative. This study’s purpose was to examine the effect of a mail-mediated intervention, based on self-determination theory, on exercise behavior in adults. Self-determination theory suggests that social contexts promoting a sense of autonomy, competence, and relatedness will increase internalization of behavioral regulation and lead to more self-determined forms of motivation associated with more positive outcomes. A positive relationship between internalization of exercise behavior regulation and actual exercise levels has been supported, however, the manipulation of social contexts to elicit greater internalization has received little attention. Additionally, mail-mediated interventions are particularly advantageous due to their relative low-cost and potential for wide distribution. Participants were 185 adults initially classified in the preparation stage of exercise behavior and randomly divided into intervention-only (INT), intervention-plus-booster (INTB), and control (CONT) groups. The INT and INTB groups received a four-page intervention packet of printed materials consisting of cognitive and behavioral strategies designed to increase perceptions of autonomy, competence, and relatedness. The CONT group received an American Heart Association exercise and health facts packet of similar length and format. After 1 month, the INTB group received a booster postcard re-emphasizing the focal points of the initial intervention packet. All participants completed psychometrically sound self-report measures of perceptions of autonomy, competence, and relatedness, exercise behavior regulation, and exercise behavior at baseline, 1 month, and 2 months. A 3 (group) X 2 (gender) X 3 (time) RM ANOVA revealed that all groups significantly increased exercise behavior over the 2-month period, however no significant interactions were detected. A trend of increased exercise behavior (p=.06) for females in the INTB group from 1 month to 2 months emerged that was not seen in INT and CONT females. A 3 (group) X 2 (gender) X 3 (time) RM MANOVA conducted for perceptions of autonomy, competence, and relatedness, revealed no significant interactions. Structural equation modeling used to examine the sequencing of relationships among variables provided partial support for self-determination theory and suggested that perceptions of competence mediated the relationship between internalization of exercise behavior regulation and exercise behavior. The lack of significant treatment effects may have been due to relatively low compliance rates reported by participants in completing intervention worksheets, or participants’ initial stage of exercise. Results do suggest, however, the potential usefulness of minimal contact follow-ups to initial intervention efforts in increasing exercise behavior as we attempt to meet national health objectives. Keyword(s): exercise/fitness, physical activity, research