When the major response domains in the rat olfactory bulb that are evoked by odorant enantiomers are compared, some of these odorant pairs do not show significantly different activity patterns. Such pairs are not spontaneously discriminated in a behavioral test. We show here that even these similar odorants appear to evoke different activity patterns when every data point in a glomerular activity array is compared. These odorants also can be discriminated if they are subjected to differential reinforcement. These data suggest that the method chosen to assess olfactory discrimination will reveal different olfactory capabilities of rats. The small differences in glomerular activity that probably exist between any pair of odorants may serve as a basis for odor discrimination when rats are differentially reinforced, thereby establishing the remarkable limits of rat olfactory perception. At the same time, the major differences in glomerular responses appear to serve as the normal basis for spontaneous odor discrimination.