From the theoretical reference frame of constructivism, much of the rhetoric and positive research findings in support of microprocessor based laboratory (MBL) activities can be interpreted in terms of the increased opportunities for student-student interactions and peer group discussions about familiar and discrepant events. However, the rhetoric is not widely matched by practice. This research aimed to increase understanding of how MBL activities specifically designed to be consistent with a constructivist theory of learning support or constrain student construction of understanding. It was conducted in a Year 11 physics class taught by the first author. Seven activities relating to kinematics were prepared in predict-observe-explain format. Data sources included video and audio recordings of students and teacher during each laboratory session, computer records of all data sets recorded by students, students' written notes and reports, semi-structured interviews with selected students, and the teacher's reflections on each session. Analysis of students' discourse and actions revealed many instances where students' initial understanding of concepts of displacement, velocity and acceleration were challenged by the data presented on the computer screen, and their negotiation of new understanding was mediated in multiple and subtle ways by the computer display. Students invented numerous techniques for manipulating data in the service of their emerging understanding. Recommendations are made for development of appropriate pedagogical strategies incorporating MBL activities which will likely catalyse student construction of understanding.