(1) Female rats with complete transections of spinal cord at low thoracic levels were not able to perform the lordosis reflex. (2) Females with bilateral transections of dorsal columns, dorsolateral columns or entire dorsal half of spinal cord performed lordosis in a normal way. (3) Bilateral transection of fibers in the ventromedial columns, added to transection of dorsal columns or entire dorsal half of the cord still allowed lordosis to be performed. (4) Large bilateral transections of the anterolateran columns caused severe loss in the strength and frequency of the lordosis reflex. Effective transections often included some ventromedial or dorsolateral column fibers, and were accompanied by abnormalities of locomotion. (5) We conclude that supraspinal control is required for the normal lordosis reflex, and that fibers necessary and sufficient for lordosis run in the anterolateral columns. These fibers are likely to be dispersed throughout the anterolateral columns, since large transections were required to eliminate lordosis, and also to be involved in (or interspersed with other fibers involved in) other aspects of motor control. Candidate ascending systems are the anterolateral fibers of Mehler. Candidate descending systems are the lateral vestibulospinal and reticulospinal tracts.