Several lines of reasoning suggest that the phosphorylated axonal form of the neurofilament subunit NF-H is likely to be released from damaged and diseased neurons in significant amounts. Detection of this protein in serum or CSF might therefore provide information about the presence and degree of neuronal loss. We therefore developed a sensitive NF-H ELISA capable of detecting picograrn quantities of phosphorylated NF-H (pNF-H). This assay showed that soluble pNF-H immunoreactivity is readily detectable in the sera of adult rats following various types of experimental spinal cord injury (SCI) and traumatic brain injury (TBI), but is undetectable in the sera of normal animals. Here we describe details of the time course and extent of serum pNF-H expression following experimental SO and TBI. Following SCI, serum pNF-H showed an initial peak of expression at 16 h and a second, usually larger, peak at 3 days. Following TBI, lower levels of serum pNF-H were detected with a peak at 2 days post-injury. We also show that the higher levels of pNF-H released from injured spinal cord as compared to brain are in line with the similar to 20-fold higher levels of pNF-H present in spinal cord. These findings suggest that serum levels of pNF-H immunoreactivity may be used to conveniently monitor neuronal damage and degeneration in experimental and presumably clinical situations. (c) 2005 Elsevier Inc. All rights reserved.