The antinociceptive effect of dipyrone, a nonsteroidal antiinflammatory drug, was studied in a series of experiments employing tail-flick and hot-plate models and the abdominal constrictor test. The drug was given via intracerebroventricular (ICV), intrathecal (IT) or subcutaneous (SC) routes, Dipyrone exhibited no analgesic activity in the tail-flick and hotplate tests while it inhibited the number of stretches in a dose-dependent manner. The antinociceptive effect of dipyrone administered by the ICV and IT routes was almost completely reversed by naloxone treatment. The same procedure attenuated but not-completely inhibited the dipyrone action induced by SC administration. Histopathological examination revealed that IT dipyrone application produces no significant lesion in the spinal cord. The results suggest that dipyrone may exert a central antinociceptive action reversed by naloxone.