The corticospinal tract is a prominent descending pathway in rodents which is thought to be involved in motor control. The purpose of this study was to investigate whether the lesions of the corticospinal tract affected overground locomotion in rats using a method of assessment which is relatively novel in this field, the evaluation of ground reaction forces. Ground reaction forces, i.e. the forces acting through the limbs on the ground, are a non-invasive, sensitive and quantitative method with which to assess gait compensation and the function of individual limbs during locomotion. We compared the three-dimensional ground reaction forces produced by locomoting rats with a unilateral corticospinal tract transection to those of control rats. Corticospinal-lesioned animals showed transient locomotor deficits 24-48 h after the lesion which quickly recovered to normal symmetrical locomotion. The initial locomotor deficits consisted of an asymmetric gait during which abnormal braking forces were produced during the dual contact time of the forelimb contralateral to the lesion (the impaired forelimb) and the ipsilateral (unimpaired) hindlimb. Normal forces were produced during the dual contact time of the ipsilateral (unimpaired) forelimb and the contralateral (impaired) hindlimb. The presence of the initial deficits may be a result of inflammation in the area of the lesion or may reflect a loss of normal locomotor contribution of the corticospinal tract. If the latter, the nature of the locomotor deficit suggests that (a) the forelimb may be more influenced by corticospinal lesions than is the hindlimb and/or (b) that the asymmetric gait produced is a general compensatory response to unilateral CNS injury in a quadruped. Complete recovery from corticospinal tract transection provides unequivocal evidence that input from the corticospinal tract is not essential for normal overground locomotion in the rat.