To examine the effect of a mail-mediated intervention, based on self-determination theory, on adultsexercise behavior. The study was a randomized control trial conducted over a 2-month period. Of the initial 185 volunteer participants, 126 (68.1%) completed questionnaires at baseline, 1 month, and 2 months. Participants in intervention-only and intervention-plus-booster groups received a mail-delivered packet containing strategies designed to promote perceptions of autonomy, competence, and relatedness regarding exercise. Those in a control group received an American Heart Association physical-activity facts packet. After 1 month, those in the intervention-plus-booster group received a booster postcard, reiterating the main points of the initial intervention packet. Exercise behavior was the primary outcome variable. Perceptions of autonomy, competence, and relatedness were evaluated as mediating variables. Separate 3 (group) x 3 (time) repeated measures analyses of variance conducted for men and women revealed that for women, all three groups significantly increased exercise levels over the 2-month period. No significant interactions were found regarding the influence of the intervention on the mediating variables. A process evaluation indicated a lack of compliance regarding completing intervention-packet worksheets. Findings suggested that more intensive interventions and greater fidelity of treatment may be needed to evidence change in exercise behavior.