The Substance Education Program of the Iowa Department of Education began a series of tri-annual studies of youth substance behaviors, attitudes, and knowledge in 1975, with later data collections in 1978, 1981, 1984, 1987, and 1990. This report presents the normative data from the 1990 survey which was administered to approximately 7,000 students in grades 6, 8 , 10, and 12, and makes relevant comparisons with earlier findings. A brief introduction is followed by a section on procedures which explains the selection of district and student samples. The next section describes findings based on students' self-reported use and nonuse during the fall months of 1990. For each of five categories (alcohol, tobacco, marijuana, other drugs, and polysubstance use), data are presented on age of onset of use and on the frequency of use by grade level, sex, and district size. The next section focuses on external influences on substance use and nonuse behaviors, looking specifically at the influence of the community, family, peers, and the school. A section on personal variables assesses internal influences on substance use or nonuse and presents data relevant to conforming and deviant behaviors. A summary and recommendations section concludes the document. The findings presented in this report on substance use and attitudes among Iowa youth shows the continuation of a 12-year decline in the use of alcohol, tobacco, and other drugs among Iowa teenagers. Eighty-five data tables are included. (NB)