The attractiveness of computer-based tests (CBTs) is due largely to their capability to expand the ways we conduct testing. A relatively unexplored application, however, is actively using the computer to reduce construct-irrelevant variance while a test is being administered. This investigation introduces the effort-monitoring CBT, in which the computer monitors examinee effort (based on item response time) in a low-stakes test and displays warning messages to those exhibiting rapid-guessing behavior. The results of an experimental study are presented, which showed that an effort-monitoring CBT increased examinee effort and yielded more valid test scores than a conventional CBT. Thus, unlike previous research that has focused on identifying rapid-guessing behavior after it has occurred, the effort-monitoring CBT proactively attempts to suppress rapid-guessing behavior. This innovative testing procedure extends the capabilities of measurement practitioners to manage the psychometric challenges posed by unmotivated examinees.