This study was undertaken to explore the nature of impaired postural stability following physical exercise. Nine healthy subjects (aged 24 +/- 3 years) were subjected to 30 min of treadmill walking or running on two separate occasions. Walking and running speeds (1.9 to 2.2 m/s) were chosen to induce equal energy expenditure. During treadmill exercise, kinematic analysis of head movement was performed. Prior to and immediately following exercise, postural sway was evaluated as mean velocity of centre of pressure (VCOP), measured on a force plate during 30 s quiet stance with eyes open or eyes closed. The results indicate that exercise increased two-dimensional postural sway by 9 - 19 % in eyes open, but not in the eyes closed condition. The deteriorating effect of exercise was both evident in the sagittal and in the frontal plane, although the effect on the former was slightly more transient. Post-exercise sway values were significantly higher after running than after walking. Kinematic analysis of head movement confirmed larger vertical displacement and acceleration pattern in running compared to walking. It is concluded that exercise of moderate intensity deteriorates visual contribution to postural stability. The effect is evident as an initial destabilisation in the sagittal direction and a less transient loss of latero-lateral stability. Running tends to disturb postural stability more than walking, possibly due to more excessive head movement and disturbance of vestibular and visual information centres.