The effectiveness of an intervention based upon the theories of Ajzen and Triandis was evaluated among 698 junior and 306 senior high school students. The intervention to juniors was offered by senior students who were trained during a course integrated into the school curriculum. Respondents in the control and experimental groups completed a questionnaire at baseline and 9 months after the program. Compared to junior respondents in the control group, those in the experimental group positively modified their attitude, perceived behavioral control, personal normative beliefs, perceived role beliefs, anticipated regret and intention with respect to postponing sexual intercourse and with respect to condom use, as well as perceived self-efficacy to negotiate both behaviors. Compared to senior respondents in the control group, those in the experimental group showed a significant positive modification of all the above variables except perceived behavioral control (indirect measure), anticipated regret and intention with respect to postponing sexual intercourse. At post-test, seniors in the experimental group were more likely to use condoms on a regular basis than those in the control group. Program effects occurred among both sexes, but a few differences in response were observed among males and females. Results suggest this type of theory-based program is effective in modifying psychosocial variables related to postponing sexual intercourse and related to condom use among adolescents. Personal involvement in designing intervention appears to be effective in modifying the behavior of peer educators. (Contains 1 figure and 7 tables.)