AbstractThis paper studies attempts to change teachers' thinking towards a construc‐tivist perspective within science education. The contexts surrounding the research are important: ideas of critical constructivism, critical action research and the work of Paulo Freire are brought to bear on teacher education in Recife, northeastern Brazil. A professional development course was designed to explore with secondary science teachers some elements of constructivist thinking, and to shape action research projects within their own classrooms. The course design is described, along with some of the responses of the participants both during and after they undertook their projects within their secondary schools. The responses highlight the tensions of trying to innovate with ‘non‐traditional methods’ within traditional situations, the advantages and drawbacks of ‘freeing up’ classroom discussion and authoritative teaching.