The newly identified neuropeptide nociceptin/orphanin FQ (NOC) was measured in different rat brain areas related to the descending anti-nociceptive pathways and compared to two opioid peptides, dynorphin B (DYN B) and Met-enkephalinArgPhe (MEAP). Two experimental models of chronic nociception, one neurogenic and one inflammatory, used in this study, reveal how different pathological conditions may influence these endogenous systems. Nerve injury is induced by ligation of the sciatic nerve and inflammation by a carrageenan injection in the gluteal muscle, 2 weeks prior to decapitation. Selected brain areas were dissected out and frozen. NOC-, DYN B- and MEAP-like immunoreactivity (LI) is determined by radioimmunoassay. Nerve injury increased the NOC-LI levels in the cortex cinguli, DYN B-LI levels in the dorsal and the ventral part of the spinal cord, whereas a decrease in the MEAP-LI levels is seen in the dorsal part of the periaqueductal grey (PAG). After inflammation, the NOC-LI levels increased in cortex cinguli, hypothalamus and in the dorsal spinal cord, whereas DYN B-LI levels increased in the dorsal part of the PAG. A general increase in MEAP-LI levels is found after inflammation in all analyzed brain areas except in hippocampus. In conclusion, increased levels of NOC-LI were found in cortex cinguli in both treatment groups and in hypothalamus and spinal cord following carrageenan treatment. The changes in the NOC-LI concentrations were not parallelled by changes in DYN B-LI and MEAP-LI, suggesting that NOC and opioid peptides elicit different reactions in the systems of nociception/antinociception.