We report findings from a case study of co-operative, group-based assessment in Pharmacology for second-year undergraduates at The University of Queensland, Australia. Students enrolled in the 2005 Bachelor of Science and 2006 Bachelor of Pharmacy degree programs, were early users of the university's new Collaborative Teaching and Learning Centre (CTLC), a specialised social learning space. A pre-existing, traditionally taught Computer Assisted Learning (CAL) module on the topic of Drug Dependence was common to the courses for Science and Pharmacy students. We describe how this module was redesigned as part of a 3-hour, co-operative assignment weighted as 5%. Details of the co-operative learning principles, the teaching strategy and the assessment design are documented. Most students achieved high academic results: Science (2005) mean [plus or minus] SD = 79.5[plus or minus]14.8%, n = 232; Pharmacy (2006) mean [plus or minus] SD = 83.3[plus or minus]13.6%, n = 186. From post-task opinion surveys and focus group interviews, both student cohorts perceived that CTLC facilities and the group work helped them learn about Drug Dependence. These data confirmed that most students felt they worked co-operatively to complete the assessment. Time and understanding the new task were the main reported stressors. (Contains 3 figures and 1 table.)