Kelatorphan (20 mg/kg i.v.) increases ventilation and there is no obvious difference between arthritic and non-arthritic rats, in arthritic rats. A slight (116%) but significant increase of respiration is also produced by Kelatorphan (20 mg/kg, n=6) 10±15 min after administration, in pentobarbital-anesthetized rats. Kelatorphan (10±20 mg/kg i.v.) increases minute-volume, in normal awake rats. The increase in ventilation is due to a dose-dependent increase in breathing frequency. The administration of Kelatorphan alone (50 μg) could cause a strong enhance of intact [3H]enkephalin content corresponding to 80±11% of total recovered radioactivity[2]. The effects of Kelatorphan are not antagonized by pretreatment with a small dose of naloxone (0.2 mg/kg i.v., 15 min before Kelatorphan), but a larger dose (1 mg/kg) significantly antagonized Kelatorphan (20 mg/kg) at 5 and 10 min in awake rats[3].