This study examines professional development activities for students and teachers in two Dutch professional development schools (PDSs). The PDSs consist of a partnership between the kindergarten classes of the elementary school and accompanying playgroup (with children aged 4-6 and 2.6-4, respectively), a teacher's training college and a research group and educational service agency attempting to improve the effects of early childhood education programmes. The professional development activities encompass the quality of adult-child interaction during instruction and daily routines. All participants learn from each other how to support and challenge children's language and reasoning skills by attending joint courses, observing each other and giving feedback. Rating scales of interactive skills were completed twice during one year by the participants themselves, colleagues and an observer. Also, the participants evaluated the interventions used. Findings indicate that although the subjects became more aware of the importance of adult-child interaction, only kindergarten teachers grew in interactive skills. (Contains 4 figures and 2 tables.)