The GABA C receptor/channel ρ 1 subunit plays an important role in the inhibitory pathway and sensory processing in the retina and spinal cord. Although it was suggested that the ρ 1 subunit plays a role in olfactory sensations, the precise role of the ρ 1 subunit in olfactory sensory function is still not clear. In the present study, we report that olfactory function was significantly altered in ρ 1 subunit knockout (rho1 −/− ) mice compared to its wildtype counterpart. The ρ 1 subunit mRNA, detected by reverse transcription (RT)-PCR experiments, was expressed in the olfactory bulb of wild-type mice. Expression of ρ 1 subunit proteins in the olfactory bulb was detected by immunohistochemistry in mitral cells in the mitral cell layer. Neither mRNA nor proteins of the ρ 1 subunit were found in olfactory bulb neurons in rho1 −/− mice. Alterations of olfactory function in rho1 −/− mutant mice compared to their wildtype littermates were examined by olfactory behavioral test. We found that sensitivity to the smell of citral odorant in rho1 −/− mice was significantly greater compared to that of wildtype mice. Our results indicate that the GABA C ρ 1 subunit acts in olfactory bulb neurons as an inhibitory modulator that affects the process of olfactory signaling transmission.