Nitric oxide (NO) plays a role in the secondary damage after spinal cord (SC) injury. NO is produced by the activity of two classes of enzymes: calcium-dependent constitutive nitric oxide synthase (NOS) and calcium-independent inducible NOS. To determine the time course of both NOS activities after SC injury, 50 Wistar rats were submitted to severe SC contusion. NOS activities were assayed at the site of SC injury at several times after lesion. Results showed a significant increase of 138 and 96% in the constitutive NOS activity at 4 and 8 h after the lesion, respectively, as compared to sham-operated rats. iNOS activity was increased 72 h after lesion by 103% (P<0.05). In conclusion, both isoforms of NOS increase their activity at different time periods after SC injury.