Quaternary derivatives of naloxone and other compounds are assumed not to enter the central nervous system following systemic administration. We report that i.p. naloxone methylbromide (5 mg/kg) completely reversed the antinociceptive effect of systemically administered morphine (6 mg/kg) in acutely spinalised rats, although it had no effect in the same animals prior to the transection. Naloxone hydrochloride was effective both before and after transection. Nuclear resonance spectra confirmed the purity of both compounds. These results suggest that acute spinal transection allows rapid entry of quaternary naloxone into the spinal cord. Quaternary compounds therefore may need to be used with caution in spinalised animals.