Objective: To explore dynamical changes of nitric oxide synthase (NOS) activity in damaged spinal cord tissue at early phase of spinal cord injury and its cell derivation. Methods: NOS activity was determined by the rate of conversion of 3H-arginine to 3H-citrulline in damaged spinal cord tissue from 0 to 60 min after spinal cord compression, and immunohistochemistry was applied to study NOS derivation. Results: The NOS activity increased sharply after spinal cord compression injury, reached the highest at 5 min, and then decreased rapidly to normal level in 1 h. NOS-Ⅰ immunoreactive interneurons were identified mainly in the intermediolateral cell column, around the central canal and in dorsal and ventral horn, but there were no cells with NOS-Ⅲ immunoreaction in spinal cord tissue. Conclusion: There is a sharp transient increase in NOS activity during the first hour of spinal cord compression, the derivation is mainly interneurons in spinal cord.