Actin as a component of the cytoskeleton is familiar as a protein which possibly mediates contraction in nonmuscular cells. In 20 mouse kidneys, the distribution of intracellular actin in the collecting tubule system was investigated by means of the anti-actin PAP method. It could be shown that the positive PAP reaction increases from the proximal to distal up to the collecting tubules in the region of papilla. Stiffening, stabilization and contraction functions are discussed with regard to the biology of the collecting tubule actin.