The action of opioid on the hyperpolarization-activated cation current (Ih) in substantia gelatinosa neurons were investigated by using whole-cell voltage-clamp recording in rat spinal brain slices. Hyperpolarizing voltage steps revealed slowly activating currents in a subgroup of neurons. The half-maximal activation and the reversal potential of the current were compatible to neuronal Ih. DAMGO (1μM), a selective- opioid agonist, reduced the amplitude of Ih reversibly. This reduction was dose-dependent and was blocked by CTOP (2μM), a selective μ-opioid antagonist. DAMGO shifted the voltage dependence of activation to more hyperpolarized potential. Cesium (1 mM) or ZD 7288 (100μM) blocked Ih and the currents inhibited by cesium, ZD 7288 and DAMGO shared a similar time and voltage dependence. These results suggest that activation of μ-opioid receptor by DAMGO can inhibit Ih in a subgroup of rat substantia gelatinosa neurons.