Levels of mRNAs coding for prodynorphin (Pro-Dyn) and proenkephalin (Pro-Enk) as well as the levels of immunoreactive (ir)-dynorphin (Dyn) and (ir)-Met-enkephalin (Met-Enk) were measured in the spinal cord of rats, 65 h following transection or injury of the spinal cord at the T6 spinal segment. Levels of both Pro-Dyn mRNA and of ir-Dyn were significantly increased between 60 and 150%, above control levels in the whole spinal cord, whereas those of Pro-Enk mRNA and of ir-Met-Enk remained unchanged. The increase in spinal levels of Pro-Dyn mRNA were highest in the areas close to the side of transection and indicate an involvement of the Pro-Dyn opioid system in the response to spinal injury and transection.