Online quizzes were introduced into a large Medical Physiology class to provide students with formative assessment before midterm and final summative examinations. Use of unsupervised online quizzes was chosen to provide a flexible supplementary learning tool for students without overwhelming a small faculty. Several quiz models were applied, which varied in the availability of course credit points for participation and performance. The aims of the study were to investigate if participation in formative assessment was associated with improved course outcomes, if offering incentives for completing quizzes affected student participation, and if quiz performance was predictive of summative examination outcomes. Results showed that students who elected to use online quizzes performed better in summative examinations. Offering course credit of between 0.5% and 2% per quiz increased student participation. However, evidence was found for widespread inappropriate use of unsupervised online quizzes when incentives for participation were applied. Predictive validity of online quizzes could be demonstrated when comparing the first of several quiz attempts with subsequent summative examination scores.