This study was undertaken to assess the sensitivity of nitrous oxide (N2O) analgesia to antagonism by intrathecally (IT) and intracerebroventricularly (ICV) administered antagonists selective for kappa- and mu-opioid receptors. Male ICR mice were pretreated IT or ICV with the kappa antagonist nor-binaltorphimine (nor-BNI), 1 or 50 nmol, respectively, or distilled water (control), then exposed to N2O (50% or 75% in oxygen). Compared with IT control mice, IT nor-BNI-pretreated mice responded with significantly less analgesia. Compared with ICV control mice, ICV nor-BNI-pretreated mice also showed markedly reduced analgesic response. Other mice were pretreated IT or ICV with either the selective and irreversible mu antagonist beta-funaltrexamine (beta-FNA, 5.0 micrograms) or distilled water (control). When exposed to N2O 24 h later, beta-FNA-pretreated and control mice exhibited comparable analgesic responses. These preliminary results suggest that N2O analagesia in mice may involve spinal and supraspinal kappa-opioid receptors but not mu-opioid receptors.