This study reports the effect of teachers' professional development in a two-year MEd program of High School Science Education, held from October 2010 until March 2012. Twelve out of the 33 teachers enrolled in the program participated in the study. The main research questions were: what are the teachers' perceptions of their own work and teaching styles at the beginning of the program; and to what extent and how does studying in the program influence these perceptions and teaching styles? In both years, the participants were interviewed and then observed teaching. The main findings were: that all of the teachers testified that their studies in the program influenced their teaching and their teaching perceptions, and complained about the constraints that prevented them from applying the new perceptions and ideas they acquired during their studies; and that five teachers remained characterized by a teacher-centered teaching style, as in the first year, while seven teachers displayed a considerable positive change toward a student-centered teaching style. It seems that there are three main factors operative in how the MEd program influences the teachers' beliefs and teaching practices: their teaching discipline; the way teachers judge their status; and the resources available at school.