6 experienced orienteers were subject to a Vo2max treadmill test, two days prior to undertaking two tests of visual perception. One test was conducted while the subjects were in a rested state while the other was conducted while they were under a state of fatigue. Fatigue was defined as a state in which the subjects were working at or above their anaerobic threshold which had been determined previously from their VO2max test. the tests in both the fatigue and rest condition were of a similar nature, that is, the subjects were presented slides of orienteering checkpoints at regular intervals followed by a slide showing a set of questions which the subjects had to answer verbally. Two sets of slides were employed and these were approximately counterbalanced between both subjects and conditions. Points were awarded for the correct answers and the two conditions were then compared. The Wilcoxon test for two correlated samples was used and showed a significant difference between the fatigue and rest scores at p < 0.05. The data suggest that under the influence of fatigue, an orienteer's ability to perceive visual information is greatly impaired.