It has long been established that pupillary responses provide a valid and reliable window on the "intensity" of mental activity or cognitive effort (Hess & Polt, 1964; Kahneman & Beatty, 1966). As these responses are routinely and noninvasively measured during eye-tracking, they constitute a promising tool for the study of the cognitive mechanisms underlying skilled performance. Specifically, larger pupil diameter during equiluminance reflects increased attentional resource allocation. In eye-tracking research, the "quiet eye" (QE) has consistently shown to he a key predictor of perceptual-motor expertise (Mann, Williams, Ward, & Janelle, 2007; Vickers, 2009). Unfortunately, despite an abundance of QE, research, there has been a dearth of theorizing on the specific purpose of QE or the mechanisms that underlie this distinctive pattern of gaze behavior. Therefore, the current study aims to tackle this gap in the literature by measuring the timing and magnitude of the cognitive load during golf putting using pupillometry. Participants consisted of 24 golfers undertaking 2 blocks of 10 putts-easy (1.83 m) and more difficult ones (3.66 m). Results indicate that peak pupil dilation directly corresponds to the onset of QE. This finding illustrates that QE onset is the most cognitively intense time for skilled golfers. Finally, results revealed that the magnitude of pupillary responses was greatest for all golfers (high- and low-handicap groups) from the moment of QE onset through to ball contact, showcasing that putting is a mentally demanding task. The theoretical significance of these results is discussed and suggestions are provided for future research.