Changes in corticospinal excitability following fatiguing exercise have been studied with transcranial stimulation techniques (Gandevia, 2001; Todd et al. 2003). Cortical and spinal excitability changes have been examined for their contributions to central mechanisms of fatigue. The production of a set level of force following fatigue requires an increase in voluntary drive and an associated increase in the perception of the effort. Using transcranial magnetic stimulation (TMS) we examined changes in corticospinal excitability and the performance of an arm flexion force-matching test up to 1 hour post-fatiguing exercise. Ten healthy volunteers participated in the study (3 females, 7 right handed, mean age 31 yrs). One week prior to testing, subjects participated in 3 training sessions of a force-matching task. The task required accurate production of isometric force (40% of maximum voluntary contraction (MVC)) by forearm flexion with the elbow positioned at 90 deg on a table. The training involved production of the target force with and without (eyes closed) visual feedback of force levels. The force-matching test required 15 consecutive attempts at producing 40% MVC with eyes closed and the mean force and coefficient of variation was determined before and up to 1 hour post-exercise. TMS was applied over the left motor cortex to the site evoking maximal responses in the right side forearm flexors. Stimulation was administered using a 70 mm figure of eight coil (rapid MagStim, MagStim Co. UK) at stimulus intensity 120% of resting motor threshold. The motor-evoked potentials (MEPs) were recorded by standard surface electromyography from right m. biceps brachii and brachioradialis, and were monitored before and at regular intervals post exercise. The exercise consisted of rapid, repetitive full arm flexion- extension, administered on the first day without weights (for 3.5 min), or 2 days later, with a weight (40% of MVC) until exhaustion (< 3.5 min). There was a significant (8%) reduction in MVC immediately after fatiguing exercise (t test, p<0.01). There was a small decrease in relaxed MEP amplitude following fatiguing exercise at post 5 min, which did not reach significance. The average force produced in the force-matching tests post fatiguing exercise was 10.5% higher than the force produced after the non-fatiguing exercise, and this difference was significant (repeated ANOVA, p<0.02). The coefficient of variation for fatiguing exercise (11%) was significantly higher than for non-fatiguing exercise (8%), (t test, p<0.02). Our findings suggest that the perception of force may be altered by fatiguing exercise, so that more force is produced when subjects must complete a force-matching test. A possible interpretation is that afferent information following fatiguing exercise is impaired and could account for a change in the perception of effort.