Systemic administration of naloxone usually produces either hyperalgesia or no 
change in nociception depending on the animal species used and/or the pain test 
employed. This study, however, demonstrates that naloxone produces a 
dose-dependent analgesia in the formalin pain test using an inbred strain of 
albino mouse. Female BALB/c, C57BL/6 and CD1 mice were injected subcutaneously 
with naloxone HCl in saline (0.1 10.0 mg/kg) or saline alone, and tested for 
analgesia using the formalin test. Naloxone produced a statistically significant 
dose-dependent analgesia in the BALB/c mice, with an ED50 of 0.24 mg/kg and 
almost total analgesia at doses of 1 mg/kg or greater. No changes in pain 
behaviour were observed in the C57BL/6 or CD1 strains of mice. We believe this 
to be the first report of analgesia following administration of doses of 
naloxone normally used for opioid antagonism. To determine if this effect was 
specific to the formalin test, the 3 strains of mice were injected 
subcutaneously with naloxone HCl and tested in the tail-flick test. Naloxone had 
no analgesic action in this test in any of the strains.