Interplay of secondary pre-service teacher content knowledge (CK), pedagogical content knowledge (PCK), and attitudes regarding scientific inquiry teaching within teacher training To investigate quantitatively the longitudinal relationship between knowledge and attitudes on scientific inquiry teaching, 121 pre-service teachers for teaching biology at the secondary level in Germany and Switzerland were assessed with respect to their content knowledge (CK), pedagogical content knowledge (PCK) and attitudes on teaching scientific inquiry. As part of the teachers' attitudes we measured cognitive beliefs, affective states and self-efficacy. The assessments were conducted during a training session on peer-coaching as a tool for teacher development and scientific inquiry teaching as a method and activity that leads to the development of scientific knowledge. Data of both knowledge tests were IRT-scaled and combined with the data from the attitude questionnaire. To examine the relationship between the different measurements we applied structure equation modelling. The analysis demonstrates that the PCK acquired after the training on teaching scientific inquiry predicts subsequent attitudes. In contrast, prior CK does not affect subsequent attitudes regarding teaching scientific inquiry. Moreover, prior attitudes do not substantially predict subsequent PCK and CK. In addition, prior teaching experience in biology explains individual differences, and self-efficacy attitudes predict changes in attitudes with respect to teaching scientific inquiry by the end of training. To foster positive attitudes toward scientific inquiry teaching, teaching education programs should be designed to broaden PCK.