Significance: Parental and peer social support (SS) is an important component in the adoption of an active lifestyle, yet has a differential effect based on who provides the support. Self-efficacy (SE) is also essential, consists of various dimensions, and is likely to influence activity levels in unique ways depending on the type. Prior work has focused on collective measures of parental SS and SE, which limits the ability to determine if specific providers of SS and types of SE are more/less important to being active. Methods: All high school girls (N = 258, 15.5±1.2 years) enrolled in physical education in one rural high school responded to a questionnaire assessing the following: physical activity (YRBS, 2005) - previous 7 day vigorous activity and total activity performed for a minimum of 60 minutes each day, past months sports participation, and cardiovascular fitness (PACER); social support (Prochaska et al., 2002) - maternal, paternal, and peer support for activity; and self-efficacy (Saunders et al., 1997) - Support Seeking, Barriers, and Competing Activities. Using height (cm) and weight (kg), girls were classified as at-risk-of or overweight (AROW, 37.7%) according to sex-specific BMI percentiles. Items for the SE subscales were summed and used as observed variables in the structural equation model analysis (Mplus, v. 3.1). The direct and indirect effects of SS through SE and the direct effects of SE on the latent construct activity (4 indicators) were modeled. Age, BMI (dummy coded as AROW = 1, other = 0), and TV and homework (amount of time past 7 days) were used as covariates. Results: Model fit was: c2 (225, N = 259) = 335.83, p < .05, CFI = .952, TLI = .938, RMSEA = 0.044. Peer SS and Barrier SE had direct positive effects on activity, with Peer SS exhibiting an indirect effect through Barrier SE. Older girls reported less SS from all sources. Overweight girls reported lower levels activity and fathers' SS. TV was inversely and homework positively related to activity levels. Overall the model explained 79.2% of the variance in activity. Conclusions: Across all ages, but especially for older girls, enhancing existing ties with peers, while cultivating new ties or using indigenous helpers within peer groups may be efficacious at increasing, or at minimum maintaining, activity levels. Although not related to activity, lower levels of father SS observed in the AROW group indicates potential mechanisms that might be related to unhealthy weight gains. Keyword(s): high school issues, physical activity