A review of 178 sources (articles, books, and other sources from 1971 through 1980, and frequently-cited earlier sources) on rural sociology and rural mental health indicated 4 major categories of definitions of rural: not explicitly stated, verbal (qualitative), homemade quantitative, and external quantitative. Sources were summarized as to author(s), publication date, focus (rural-urban comparison, rural sociology, rural health/mental health, rural human services), basis for defining rural, statistics employed (none, narrative, descriptive, univariate, multivariate), and findings. An unstated definition was used by 43% (77 sources); 19% (33) used verbal definitions, 15% (27) utilized homemade quantitative definitions, and 23% (41) used external quantitative definitions. Of 101 sources which defined rural, 22 used multiple components, but only 5 sources using homemade quantitative and none using external quantitative definitions employed multiple criteria. Rural was defined in terms of population by 90 sources (verbal = 25, homemade quantitative = 25, external quantitative = 40), but with little consensus on population criteria. The most common quantitative external population definitions, both based on census data, were those of the Department of Commerce (Rural versus Urban) and Office of Management and Budget (Standard Metropolitan Statistical Area). Other criteria included occupation (14 sources), socioeconomic information such as education and income level (19), values (7), isolation (5), and government (4). (MH)