The effect of cauda equina constriction (CEC) on nitric oxide synthase (NOS) activity has been studied in spinal cord of the dog. CEC lasting for 2 days caused significant increase in NOS activity in white matter of lumbar segments. The enzyme activity decreased to basal levels 5 days after injury. An extreme reduction of NO formation has been seen in sacral segments at 2 day of postsurgery. At 5 day, the enzyme activity greatly increased and in gray matter significantly exceeded baseline. Quantitative measurement of NOS activity produced a rise in gray matter of thoracic segments and indicated a decrease in gray matter of upper lumbar segments. The results indicate a time‐dependent formation of nitric oxide after CEC in spinal cord segments and regions.Acknowledgements: Supported by VEGA Grants no. 2/2079/22 and 2/3217/23PS9, STAA Grant no. 51‐013002 and by NIH grants NS 32794 and NS 40386 to M.M.