Previous theory and research implicates participation in meaningful activity as an important factor in enhancing older adults' health-related quality of life. Consistent with this emphasis, a previous R01 grant completed by our study group demonstrated that an activity-based intervention (the Well Elderly Intervention) reduced declines in a wide variety of health-related parameters among low income, ethnically diverse elders. In the current four-year project, we attempted to replicate our previous result while simultaneously examining the mediating mechanisms responsible for its positive effects. The outcomes of this study will provide important new information about the process events by which activity-based lifestyle interventions influence key aging outcomes.