1. Recordings of extracellular spike responses were made from single mitral/tufted cells in the ventromedial region of the main olfactory bulb of urethan-chloralose-anesthetized rabbits. Using periodic artificial inhalations, the olfactory epithelium was stimulated with series of aromatic and aliphatic compounds systematically varying in molecular conformation. 2. Analysis of response specificity of single mitral/tufted cells for alkylbenzenes indicated that the length of the hydrocarbon side chain attached to the benzene ring plays a role in determining the specificity of excitatory spike responses. 3. For a panel of isomeric (ortho-, meta-, and para-positions) disubstituted benzenes, single mitral/tufted cells tended to be activated selectively by one or two specific structural isomer(s). For a panel that contained both alkylbenzenes and disubstituted benzenes, single mitral/tufted cells were activated by subsets of odor molecules having similar conformations. These observations suggest that the overall conformation of the aromatic compounds plays an important role in determining tuning specificity of individual mitral/tufted cells. 4. For a panel of monosubstituted benzenes with various functional groups, single mitral/tufted cells in the ventromedial region tended to be activated not only by molecules having a hydrocarbon side chain (alkylbenzenes), but also by those having a methoxy group (--O--CH3), a bromine (--Br), or a chlorine (--Cl). However, most of the neurons were not activated by those having an amino group (--NH2), a hydroxy group (--OH), nor a carboxyl group (--COOH). 5. Examination with an expanded panel of stimulus odor molecules that included both aromatic and aliphatic compounds indicated that single mitral/tufted cells show excitatory spike responses to a range of odor molecules (molecular receptive range) having similar conformations. Different mitral/tufted cells in the ventromedial region typically showed different molecular receptive ranges. 6. In mitral/tufted cells with relatively high spontaneous discharges, single neurons in the ventromedial region showed inhibitory responses to subsets of odor molecules in addition to the excitatory response to other subsets of odor molecules. The odor molecules that caused inhibitory responses in single mitral/tufted cells showed molecular conformations resembling each other. 7. The present results together with previous studies indicate that determination of the molecular receptive range properties (both excitatory extent and inhibitory extent) of single mitral/tufted cells is a useful method for characterizing individual bulbar neurons. These results further support the hypothesis that conformational parameters of ligand odor molecules play a key role in sensory processing in the main olfactory bulb.