Proteasomes are the major cytosolic protease complexes responsible for energy-dependent and extra-lysosomal proteolysis. Ubiquitinated proteins are degraded by the 26S proteasome which is composed of a 20S proteasome and two regulatory complexes. Changes in the 20S and 26S proteasome activity levels and protein abundance in spinach seeds during imbibition and germination were examined by using glycerol density gradient centrifugation. The 26S proteasome activity level decreased transiently during imbibition, reached a minimum one day after starting imbibition, and then increased again. During the period of minimal accumulation, the protein being detected with an anti-26S proteasome antibody shifted to a lower sedimentation coefficient fraction. In contrast, the 20S proteasome activity level increased during imbibition, and remained high for two days. The change in the 20S proteasome level corresponded to the change in the 20S proteasome activity level.