Few students of either gender enroll in high school physics courses, but young women are especially underrepresented. A total of 698 freshmen from five Iowa high schools participated in a study in which Fishbein and Ajzen's Theory of Reasoned Action was applied to the study of the attitudes and social support that influence decisions to enroll in high school physics, chemistry, and biology classes. Attitude toward enrollment and social support for enrollment predicted enrollment intent, with gender, academic ability, and self-efficacy explaining a small but significant portion of additional variance. Examination of beliefs underlying attitudes and subjective norms suggests that for physics enrollments to increase, students need to feel that they would be successful if they took a physics class, class activities must be enjoyable for male and female students, and parents and students must be aware of the advantages of taking high school physics.