Protein content and ATPase activities have been determined in the superficial and deep layers of the rat olfactory bulb. Protein levels, Mg2(+)-ATPase and Na+,K(+)-ATPase activities were significantly higher in the whole homogenate for the superficial layers. These differences were amplified when activity was expressed on a wet weight basis in a crude microsomal preparation isolated by differential centrifugation. Specific activities, however, showed similar values in the microsomal fractions from superficial and deep layers. The results are discussed in terms of differences in the density of neuronal processes and efficiency of K+ reuptake mechanisms.