The main purpose of this study was to examine the influence of knowledge of running length (i.e., duration or distance), or the lack of it (unknown endpoint) on the ratings of perceived exertion and estimated time to exhaustion. Fourteen men were recruited for this study. Participants performed 3 tests of a similar intensity (90 % maximal aerobic velocity) and length (100 % Tlim/Dlim). However, the exercise length was differently expressed. This was either unknown or defined in terms of duration or distance. The tests with a known length were performed in a counterbalanced order. During the tests, ratings of perceived exertion, estimated time to exhaustion, and heart rate were measured then compared by using 2-way analysis of variance for repeated measures. The results show that the ratings of perceived exertion, estimated time to exhaustion, and heart rate increased significantly with exercise duration (p < .001). Moreover, the ratings of perceived exertion were significantly different between the test with an unknown running endpoint and that with a known running distance at 40, 60 and 80 % Tlim (p < .05). Therefore, athletes perceived the run as being easier during the test with an unknown running endpoint compared to the test with a known running distance. Consequently, knowledge of running length influenced the ratings of perceived exertion probably due to teleoanticipation.