The purpose of this study was to examine teaching assistants' conceptions of the nature of teaching after they were taught a pedagogical strategy directly related to their subject matter. Four teaching assistants who taught the Biology 1201 laboratories at the University of Minnesota participated in this study which included participating in 16 hours of pre-quarter instruction and weekly 3-hour pre-laboratory instruction, and teaching a total of 10 laboratories. Their conceptions of the nature of teaching were examined using the Nature of Teaching Questionnaire and three interviews, before, during, and after the instruction and teaching the laboratories. Before the quarter, most of the teaching assistants had either subject matter knowledge and/or pedagogical knowledge conceptions of the nature of teaching. The comparison between pre- and post-experience conceptions revealed that their conceptions of the nature of teaching changed dramatically after participating in the instruction and teaching the laboratories toward a pedagogical subject matter knowledge conception. The post evaluation revealed that they showed a consistent relationship toward the use of pedagogical subject matter knowledge in the areas of assessment of student understanding and evaluation of instruction. Appendices include questionnaire and interview questions. Contains 24 references. (Author/JRH)