Ultrarunning requires extraordinary endurance, but the psychological factors involved in successful ultrarunning are not well understood. One widely held view is that fluctuations in mood may play a pivotal role in performance during endurance events. However, this view is primarily based on comparisons of mood before and after marathons and shorter running events. Few studies have explicitly examined mood changes during ultra running, and they have tended to have small samples and/or be lab based rather than field studies, perhaps due to the large timescales and difficulty recruiting large numbers of ultramarathoners. To address these weaknesses, we conducted 3 field studies designed to measure mood fluctuations during competitive ultramarathon events. In Study 1, the POMS was used to monitor mood in 14 ultrarunners during the 110 mile Hardmoors110 ultramarathon. Study 2 assessed mood with the BRUMS as 15 athletes completed the Hardmoors110, and in Study 3 the BRUMS was used to assess mood in 30 athletes running a sixty mile ultramarathon. In all 3 studies there was a significant decrease in Tension between Race Start and the 1st within-race measurement point. Vigour decreased across the race whilst Fatigue increased. Importantly, within-race measures revealed that these changes were nonlinear, such that changes were largest at the start and end of races. In both the 110 mile races Total Mood Disturbance was stable throughout the 1st part of the race, then rapidly increased until reaching asymptote during the final stage. Anger significantly increased, but only in the 60 mile races. It is argued that changes in Tension reflect the release of pre-competition anxiety, that changes in Vigour and Fatigue are related to the perception of effort which may be best understood in terms of the Psychobiological Model of endurance, and that increases in Anger may have some benefits in short ultramarathons. These data offer new insights into the dynamics of mood states during ultra-endurance events, help better understand at what point during races implementing psychological mood-regulation strategies might be optimal, and may have broader implications for the understanding of human endurance and resilience in other domains.