In the present research, we investigated the relation between changes in students' state self-control capacity and their motivational test-taking effort over the course of an achievement test. Thereby, we considered trait self-control as a major predictor of achievement-related behavior as a covariate. N = 1840 apprentices repeatedly rated their state self-control capacity and the test-taking effort they invested while working on a 140-min achievement test in mathematics and science. Using growth curve analyses, our results revealed correlated decreases in state self-control capacity and the test-taking effort invested over the course of the test. Furthermore, we found that trait self-control helped to keep state self-control capacity and test-taking effort at a higher level over the course of the test. Our results support the assumption of the process model of self-control that waning self-control capacity is reflected in reduced motivational effort. Furthermore, our findings provide evidence that self-control might play a crucial role in students' test-taking behavior in large-scale assessment studies. By modeling changes in state self-control capacity and effort investment while considering trait self-control, we provide an alternative approach for investigating self-control-dependent processes and the underlying mechanisms of self-control in achievement situations.