Research indicates that modifying teachers' beliefs about learning and teaching (i.e. teaching orientation) may be a prerequisite to changing their teaching practices. This mixed methods study quantitized data from interviews with 65 graduate teaching assistants (GTAs) from science, technology, engineering, and mathematics (STEM) fields to assess the relationship of participants' teaching experiences and available teaching support systems with changes in their teaching orientation over time. These individuals represent an important but understudied link in the STEM pipeline, because they serve as primary instructors in large, introductory science laboratory classes for undergraduates at large research universities. Mentor involvement in teaching and departmental/university training and support for teaching were significantly related to change in teaching orientation toward more student-centered beliefs. Consideration of why other factors failed to evidence a relationship with teaching orientations and recommendations for how study findings can influence policy and practice are offered.