The purpose of the present research was to identify some of the psychological processes through which changes in contextual motivation toward science courses can occur. In line with the hierarchical model of intrinsic and extrinsic motivation (Vallerand, 1997), it is proposed that much of the changes in contextual motivation toward science are induced by repeated changes in situational motivation toward science-related activities. Furthermore, situational motivation itself is determined by one's contextual motivation. Finally, contextual science motivation should predict intentions of taking future science classes and pursuing a science career. Participants were high school students engaged in science courses. A longitudinal design with 5 measurement time points was used. Overall, the results of structural equation modeling analyses supported the hypotheses.