Previous studies have shown that there are some changes in our perception of time during exercise, but the relationship between intensity level and these perceptions is unclear. Therefore, the purpose of this study was to determine the effect of exercise intensity on prospective time estimations. Twenty-two trained runners (10 male, 12 female; age 25±6 years) participated in three 30-minute treadmill runs that were perceptually regulated at rating of perceived exertion (RPE) levels of 13 (“somewhat hard”), 15 (“hard”) and 17 (“very hard”). Prospective time assessments, in which subjects estimated durations of 1, 3, 7 and 20 seconds, were obtained immediately before exercise, during (at 10 min. and 20 min.) and after exercise. A 3 (RPE) x 4 (timepoint) x 4 (estimated duration) repeated measures ANOVA was completed. There was a significant main effect of RPE level (p=.013). Post hoc tests revealed that time estimations at RPE17 were significantly lower than those at RPE13 (p=.021). The main effects of timepoint and estimated duration were not significant (both p≥.05), and no interactions were present. However, there was a trend for time estimations to decrease in all conditions as exercise progressed, with a rebound after cessation of exercise. This study showed a clear effect of exercise intensity on time perception. Specifically, the subjects perceived time to pass by more slowly as intensity increased.