The behavior of ATPase in cotyledon tissue of Phaseolus vulgaris during germination has been examined with a view to assessing the feasibility of its involvement in transport processes. Both basal ATPase activity (no cations added to the assay mixture) and Na+–K+ stimulated activity were routinely present in homogenates of the tissue but decreased during germination in a manner that corresponded very closely with the pattern of protein attenuation for the same period. Since the decreased levels of protein primarily reflect protein body digestion, this parallel behavior suggests that as the need for transport of metabolites out of the storage cells becomes less, there is a corresponding decrease in the levels of basal and cation-stimulated ATPases.Fractionation of the tissue revealed that ATPase activity is present in isolated soluble fractions, but throughout the germination period the major proportions of both basal and Na+–K+ stimulated activities proved to be particulate and were distributed among nuclear, mitochondrial, and microsomal fractions. Since both types of ATPase are present in purified cell wall and plasma membrane from this tissue, this distribution is thought to reflect in large part fragmentation of cell wall and plasma membrane incurred during homogenization. Thus the subcellular distributions of the enzymes as well as their patterns of change during germination are consistent with the involvement of a major part of the cotyledon ATPase activity in transport phenomena.