We studied the association between demographic, socioeconomic, and psychosocial factors and change in level of physical activity in 4,025 persons who were interviewed in 1965 and 1974 as part of the Alameda County study. Relative declines in physical activity were found to be associated with education level (less than a college education), race (being black or non-white [women]), low income, blue-collar occupation, unmarried status, group non-membership, social isolation, depression, personal uncertainty (women), low life satisfaction, and health perceived less than "excellent." These results demonstrate the multifactorial determination of level of physical activity.