The effect of systemic morphine (5 mg/kg) on the C-fibre evoked activity in the dorsal horn of decerebrate spinalized rats has been investigated. Activity in units recorded from lamina 5 was inhibited in a naloxone-reversible fashion by morphine. However, morphine produced variable changes in the C-evoked activity of units recorded within the substantia gelatinosa, some units showing excitation, some inhibition and some an alteration in the timing and pattern of the C-evoked activity.