The course and location of vestibulospinal, reticulospinal and descending propriospinal fibres in man are reported. The investigation was carried out on three patients with supraspinal lesions, four with transection of the spinal cord and 33 with anterolateral cordotomies. The lateral vestibulospinal tract at the medullospinal junction and in the first three cervical segments lies on the periphery of the spinal cord lateral to the anterior roots. It moves to the sulcomarginal angle in the remaining cervical segments. In the thoracic cord, it moves laterally, being traversed by the most lateral of the anterior roots. Reticulospinal fibres descend bilaterally in the spinal cord with a preponderance of ipsilateral fibres. Reticulospinal fibres in general do not form well-defined tracts, but are scattered throughout the anterior and lateral columns. They are intermingled with propriospinal fibres and with ascending and descending fibres of other systems. Most reticulospinal fibres move posterolaterally as they descend. It follows that fibres from the brainstem that enter the cord in the anterior column may be in the lateral column anterior to the lateral corticospinal tract at lower levels. Reticulospinal fibres within the lateral column lie anterior to the lateral corticospinal tract. They consist of scattered fibres between the lateral horn and the periphery, most of them in the medial two-thirds of the column. In addition, they are present in a more compact group, forming a triangle on transverse section on the periphery of the lateral column, immediately anterior to the lateral corticospinal tract. On the periphery of the anterior and anterolateral columns reticulospinal fibres descend as small groups or as a continuous band of fibres. The most medial of these reaches the sacral segments and is included in the sulcomarginal fasciculus. A compact group of fibres, shown previously to be central sympathetic fibres ending in the intermediolateral and intermediomedial cell columns, surrounds the lateral horn. They do not extend throughout the thoracic cord in all cases. Anterior to this group is another group of fibres lying on the anterolateral surface of the anterior horn. As these fibres were degenerating following a pontine lesion, they must be reticulospinal fibres. The fibres were not seen in all cases and they did not always reach the lowest thoracic segments. Reticulospinal fibres enter the grey matter in the zona intermedia and along the anterolateral and anterior surfaces of the anterior horns. Caudal to the cervical enlargement, the number of reticulospinal fibres decreases, and their place is taken by propriospinal fibres. But they are not totally replaced by the propriospinal fibres, for reticulospinal fibres continue down into the lowest sacral segments. Of the propriospinal fibres, the majority are short: descending fibres within the juxtagriseal layer are one to two segments long or less.