Within the framework of social cognitive theory, we examined the motivation of students (14-16 years old) to learn science in their introductory science courses. The students responded to a questionnaire about their intrinsic motivation, self-efficacy, and self-determination. The students also wrote essays about their motivation, and individual interviews were conducted with a representative sample of students. We found that the students' intrinsic motivation, self-efficacy, self-determination, and achievement were related. Consistent with social cognitive theory, self-efficacy was the motivation factor most related to achievement. The Advanced Placement Program (AP) aspirants were higher than nonaspirants in intrinsic motivation, self-efficacy, self-determination, and achievement. Patterns in students' essays and interviews identified inspiring teachers, career interests, and collaborative-learning activities as strong motivators. The findings suggest that science teachers should use social modeling and collaborative-learning activities to foster students' motivation, achievement, AP intent, and interest in science careers. (Contains 3 tables and 1 figure.)