We have examined the regulation of endocytosis in polarized Madin-Darby canine kidney (MDCK) cells. Using quantitative electron microscopy and biochemical measurements, we found that incubation with the tumor promoter phorbol 12-myristate 13-acetate (TPA) stimulated endocytosis of horseradish peroxidase (HRP) and ricin four- to fivefold at the apical side in MDCK cells, whereas the uptake at the basolateral membrane was unaffected. The use of several protein kinase inhibitors and TPA analogues indicated that the stimulation of apical endocytosis was mediated via protein kinase C independently of protein kinase A. This stimulation occurred even when the clathrin-dependent pathway was inhibited by acidification of the cytosol, suggesting that the TPA-stimulated uptake was associated with a clathrin-independent mechanism. Moreover, we found that TPA also stimulated recycling of ricin to the apical domain. Ultrastructural analysis of MDCK cells preincubated with TPA revealed that neither the morphology nor the size of the endosomes was altered compared to control cells. Using morphometry, no marked change in the apical plasma membrane area was detected after incubation with TPA. These data indicate that the TPA-stimulated endocytosis involved neither ruffling nor formation of macropinosomes in MDCK cells. Finally, we found that TPA also selectively stimulated apical endocytosis in the human colon adenocarcinoma cell line (Caco-2). The data suggest that protein kinase C is involved in a strong stimulation of apical endocytosis and might participate in the regulation of membrane trafficking between the apical plasma membrane and apical endosomes in polarized epithelial cells.