This paper presents an overview of the inservice model used by the National Science Foundation supported Project INLAB (Integrating Laboratory Instruction and Assessment) to challenge and change science teachers' laboratory instruction and assessment practice. The framework that guides the instructional strategies and activities of INLAB is based on the notion of "pedagogical reasoning and action," which emphasizes an intellectual basis and a process for constructing a shared understanding of teaching through reflection. Project participants were 30 middle level (grades 4-9) teachers. In a workshop setting, collaborative dialogue and reflection on current practice helped participants to know and understand the ways of teaching and assessing that lead to change in practice. Evaluation data indicate that 24 out of the 30 participants have changed their laboratory teaching and 27 out of 30 have changed their assessment practice. These findings indicate that Project INLAB provides a model for collaborating with practitioners that fosters professionalism through the establishment of a dialogical and reflective community of learners. Three appendices provide: guidelines and procedures for reflection on current practice; an alternative assessment assignment; and a teachers visiting teachers assignment. (Contains 9 references.) (ND)