The trans-contextual model (Hagger et al., 2003) suggests Self-Determination Theory and the Theory of Planned Behavior (TPB) can be integrated to interpret the influence of physical education on future physical activity behavior. Using the trans-contextual model as a framework, we designed this study to examine the extent to which the self-determined motivation in mandatory physical education predicts intention and actual enrollment in elective high school physical education classes. Participants were 572 9th graders (ages 14-16 yrs, 46.1% females). Self-determined motivation in physical education was measured using the Self-Regulation Questionnaire (Goudas et al., 1994). Intention (INT), attitude (ATT), subjective norm (SN), and perceived behavioral control (PBC) for participation in elective PE next year was measured based on the TPB scale (Ajzen, 2004). Students' actual enrollment information was obtained at the end of the semester from their physical education teachers. A MANOVA examining gender differences was significant, F (6, 565) = 17.50, p <.01, ©¯2 =.14. Males reported a higher degree of self-determined motivation than females (p<.01, ©¯2 = .08) in mandatory physical education as well as a more positive attitude (p<.01, ©¯2 = .08) and greater intentions (p<.01, ©¯2 = .06) toward future elective PE classes. Also, males' actual enrollment was higher than females' (p <.01, ©¯2 = .07). Two hierarchical regression analyses predicting the actual enrollment and INT were then conducted. The order and content of the blocks were based on the theoretical tenets of the trans-contextual model. In both equations, trans-contextual constructs were entered first, followed by gender in a separate block. Results showed that the trans-contextual model variables predicted 15% of the variance in the actual enrollment in elective PE, F (5, 566) = 77.61, p<.01. Gender was not significant. For INT, the trans-contextual model variables predicted 41% of the variance in behavior intention F (4, 567) = 96.41, p<.01. Additionally, gender added significantly to the intention, FD = 17.11, p<.01. Our findings support the significant impact of mandatory PE on students' future physical activity behavior. Given the importance of continuous participation in elective PE on health and perceived competence in physical activities and exercises (NASPE, 2004), PE teachers should exert effort to enhance students intention and actual participation rates in elective PE. Two specific recommendations are: (1) making physical education "interesting and meaningful" for promoting self-determined motivation and (2) providing "knowledge" about the benefits of participation in elective PE for nurturing positive attitude. Keyword(s): active participation, high school issues, physical education PK-12