Learning theorists have provided ample evidence supporting the use of active, student-centered, social learning environments. However, little action has been taken within U.S. university curricula to transform lecture courses so that they include such teaching methods. By adding cooperative and collaborative activities into large-lecture, introductory biology courses, I was able to measure the impacts of such active-learning strategies on student attendance and performance. I gathered data from two investigations involving 378 undergraduates from paired sections of biology, one section using activelearning activities and one not. In the first investigation, I used a mixedmethods approach to measure the effects of a cooperative pre-exam group discussion on student performance, confidence, and anxiety. In the second investigation, I used a quantitative approach to measure the effects on course attendance and performance of using scenario-based collaborative activities regularly throughout a semester. Students who engaged in cooperative preexam discussion did not show significant individual learning gains but did show an increase in confidence and a decrease in anxiety. Students who engaged in scenario-based collaborative activities showed significantly higher learning gains and course attendance. The identified gains are promising for course reform.