We used whole-cell recording to identify, for the first time, GABAergic tonic current in subpopulations of substantia gelatinosa neurons in mouse spinal cord slices. Application of the γ-aminobutyric acid type A receptor antagonist bicuculline revealed tonic currents in 49 of 73 substantia gelatinosa neurons. These tonic currents were dissected into three groups according to their diverse pharmacological properties. In the first group, furosemide failed to block the tonic current while midazolam and propofol potentiated it. In the second group, furosemide blocked the tonic current but midazolam and propofol failed to enhance the current. In the third group, furosemide blocked and 3α, 21-dihydroxy-5α-pregnan-20-one enhanced the tonic current. The presence of mRNAs for α4, α5, δ and ε subunits in the substantia gelatinosa (reverse-transcriptase polymerase chain reaction) suggest that γ-aminobutyric acid type A receptors comprising α5βxγ2, αxβxε and α4βxδ subunits are responsible for tonic currents.