Using nationally representative data from students in grades 9 to 12 from the national Youth Risk Behavior Surveys (YRBS) of 1991, 1993, 1995, and 1997, this study examined changes in high school students' participation in health risk behaviors. Ten specific health risk behaviors were identified, each of which poses potential immediate and long-term health problems. Findings show that the 1990s were a period of substantial change, some good and some bad, in students' participation in specific health risk behaviors. A reduction in sexual activity and changes in contraceptive use were accompanied by declines in the teen pregnancy rate, birthrate, and sexually transmitted disease rate. Declines in fighting and weapon-carrying parallel positive changes in associated health outcomes. Decreases in the prevalence of suicidal thoughts were not accompanied by changes in suicide attempts. Marijuana and cocaine use increased among high school students between 1991 and 1997, but more recent data indicate that the rates of substance abuse among students are leveling off or even declining. Some differences in risk taking were noted for age, grade, and gender. In general, high school students reported a shift toward less overall risk taking. Hispanic students were the exception. They did not experience the same shift towards less risk taking as other students. (Contains 4 tables, 1 box, 1 figure, and 17 endnotes.) (SLD)