Background: Previous anatomical and radiological studies of conus position with flexion and extension of the spine have had conflicting results. We previously performed a human cadaveric study with direct visualization of the conus during flexion and extension to further study this question and to potentially determine if flexion and extension of the spine during MRI may prove to be a diagnostic tool in occult tethered cord syndrome. We found no movement of the conus during flexion or extension. The present is a follow-up study using MR imaging of Rhesus (Macaca mulatta) monkeys to see if the conus moves in flexion or extension of the spine. Materials and methods: We placed three adult male Rhesus monkeys under general anesthesia into a 1.5 T Siemens MRI machine. Sagittal T2-weighted images were obtained through the lumbosacral spine during flexion and extension. The conus position was compared between studies. Results: In all animals, the conus terminated between L6 and S1, and the conus did not move from its neutral position in either flexion or extension of the spine. Conclusions: Flexion and extension of the non-human primate's spine does not result in a change of position of the conus medullaris to change position. Therefore, it is unlikely that the conus would change position during flexion and extension MRI in an adult without a tethered spinal cord. © 2009 Springer-Verlag.