We investigated the expression of standard proteasomes, immunoproteasomes, and their regulators, PA28, and PA700, in rat tissues. Immunoproteasomes (with subunits LMP2, LMP7, and MECL1) were abundant in the spleen but almost absent in the brain. In contrast, standard proteasomes (with X, Y, and Z) were highly expressed in the brain but not in the spleen. Both proteasome types were present in the lung and the liver. PA700 subunits (p112, S5a, and p45) were found in all tissues. PA28 alpha, PA28 beta, and PA28 gamma were also expressed in all tissues, except for the brain which contained very little PA28 beta. The results did not depend on rat sex or age. The cleavage specificity for peptide substrates differed greatly between brain and spleen proteasomes. Hybrid proteasomes, containing both PA28 alpha beta and PA700, were not present in the brain but in all other tissues examined.