Weinvestigated ultrastructural localization of dipeptidyl peptidase IV (DPP IV)[EC3. 4.14 5] in rat liver cells quantitatively by post embedding protein A-gold technique. In the hepatocyte, DPPIV was mainly localized on the bile canalicular surface and the lysosomal membranes, but were scarcely detectable on the sinusoid-lateral surface. Asmall numberof DPPIV wasalso detected in the trans region of the Golgi apparatus, suggesting that this part mayplay important roles in intracellular transport or recycling of this enzyme. In the endothelial cell, DPPIV existed on the whole surface of the plasma membraneand the lysosomes. In the Kupffer cell DPPIV was mainly localized in lysosomes and a few were detected on the plasma membrane.