Odorant receptors (ORs) form one of the largest gene families in the genome. However, the vast majority are orphan receptors as the ligands that activate them remain unknown. Deorphaning approaches have generally focused on finding ligands for particular receptors expressed in homologous or heterologous cells; these attempts have met with only partial success. Here, we outline a conceptually different strategy in which we search for odorant receptors activated by a known odorant. Intrinsic signal imaging of the main olfactory bulb is first used to locate activated glomeruli in vivo, followed by retrograde tracing to label the sensory neurons in the olfactory epithelium projecting to the activated glomerulus. Subsequently, single cell RT-PCR is used to reveal the identity of the odorant receptors expressed in retrogradely labeled neurons. To demonstrate the applicability of this method, we searched for candidate ORs responding to the aldehyde odorant butanal. This method may be a useful tool to decipher specific ligand--OR interactions in the mouse olfactory bulb.