BACKGROUND AND PURPOSE: To quantify the late dose-related responses of the rat cervical spinal cord to X-ray irradiations by an array of microbeams or by a single millimeter beam. MATERIALS AND METHODS: Necks of anesthetized rats were irradiated transversely by an 11 mm wide array of 52 parallel, 35 ?m wide, vertical X-ray microbeams, separated by 210 ?m intervals between centers. Comparison was made with rats irradiated with a 1.35 mm wide single beam of similar X-rays. Rats were killed when paresis developed, or up to 383 days post irradiation (dpi). RESULTS: Microbeam peak/valley doses of ?357/12.7 Gy to 715/25.4 Gy to an 11 mm long segment of the spinal cord, or single beam doses of ?146-454 Gy to a 1.35 mm long segment caused foreleg paresis and histopathologically verified spinal cord damage; rats exposed to peak/valley doses up to 253/9 Gy were paresis-free at 383 dpi. CONCLUSIONS: Whereas microbeam radiation therapy [MRT] for malignant gliomas implanted in rat brains can be safe, palliative or curative, the high tolerance of normal rat spinal cords to similar microbeam exposures justifies testing MRT for autochthonous malignancies in the central nervous system of larger animals with a view to subsequent clinical applications.