We studied the origin and mechanism of spinal cord pulsation in ten dogs in order to elucidate its clinical significance. Under general anesthesia, a 6 cm length of the animals' cervical spinal cord was exposed and the cerebrospinal fluid removed. The amplitude of spinal pulsation was then measured by means of ultrasonography in Mode M. The measurements were made after the spinal cord was cut: (1) on the cranial side; (2) on the cranial as well as caudal side, with the nerve roots and radicular arteries intact; (3) on both sides as in (2), which was then supplemented with the severance of the nerve roots and radicular arteries. It was demonstrated that, while the pulsation amplitude stood at an average of 88.0 microns before the surgical treatment, it rose to 455.0 microns in (1) and 274.8 microns in (2), but dropped to nearly zero in (3). The 5.2-fold increase in pulsation following the cordotomy on the cranial side was attributed to two factors: (1) the increased spinal mobility due to the cordotomy; (2) the elevation of blood pressure. The results of the three different degrees of spinal detachment suggested that spinal pulsation derived mainly from the radicular arteries and that its presence indicated low tonicity in the spinal cord and favorable circulation in the radicular arteries.