This purpose of this study is to investigate how the course designed based on constructivist principles has been implemented, what actions have been taken to solve problems and what thoughts have arisen in the minds of teacher candidates with regard to the constructivist learning approach. In this study, an action research was employed which emanated from qualitative research designs. The participants in this study consisted of 55 science teacher candidates from the fields of biology, physics and chemistry for secondary education enrolled on the Teacher Training Certificate Program at one of large state university in Turkey. So as to collect necessary data for the study, reflection notes written by the teacher candidates, their products and a questionnaire to elicit opinions were used. The data obtained were then analyzed by using descriptive and content analysis methods. The acquired results demonstrated that on account of teacher candidates' increased exposure to the constructivist learning environment, their knowledge and professional skills increased over time. The teacher candidates outlined in their reflections on the process that becoming better acquainted with the approach did indeed make valuable contributions to their cognitive, academic and affective development. However, they indicated that factors such as inadequate provision of equipment in classrooms, oversized classes, individual differences in academic abilities of students and the centralized examination system prevented the successful implementation of this approach in the schools.