Inclusion Criteria:

   - Men and women ages 55 and older

   - Free of any medical condition that would limit participation in independent exercise

   - Not currently engaged in a regular pattern of physical conditioning

   - Free of clinical psychopathology

   - Stable on current medications

   - Planning to remain in the geographic area through the duration of the trial

   - Able to read and speak English sufficiently to understand protocol materials

   - Can use the telephone unaided

   - Willing to accept random assignment to any study condition

Exclusion Criteria:

   - Under the age of 55

   - Currently under treatment for an acute, serious medical condition (e.g., cancer, heart
   disease, stroke)

   - Physically active on a regular basis (i.e. performing more than 60 minutes per week of
   aerobic physical activity of at least a moderate intensity)

   - Unstable and/or uncontrolled on medications for high blood pressure or depression

   - Unable or unwilling to use a telephone unaided

   - Unwilling to accept random assignment to study condition