The benefit of proactive work behaviors for performance-related outcomes has been well established. However, this approach to studying proactivity has not yet acknowledged its potential implications for the actor's well-being. Drawing on the fact that resources at work are limited and that the workplace is a social system characterized by interdependencies, we proposed that daily proactivity could have a negative effect on daily well-being. We furthermore proposed that this effect should be mediated by work overload and negative affect. We conducted a daily diary study (N = 72) to test the potential negative effects of proactivity on daily well-being. Data was collected across 3 consecutive work days. During several daily measurement occasions, participants reported proactivity, work overload, negative affect, and fatigue. They also provided 4 saliva samples per day, from which cortisol was assayed. Based on the 4 samples, a measure of daily cortisol output was produced. Multilevel analyses showed that daily proactivity was positively associated with higher daily cortisol output. The positive association of daily proactivity with bedtime fatigue was marginally significant. There was no support for a mediating effect of work overload and negative affect. Implications for theory-building on the proactivity-well-being link are discussed. (PsycINFO Database Record