Potocytosis is an endocytic pathway that utilizes glycosylphosphatidylinositol-anchored membrane proteins and caveolae to concentrate and internalize small molecules. We now report that activators of protein kinase C are potent inhibitors of potocytosis. Activators such as phorbol-12-myristate-13-acetate (PMA) inhibit the internalization of receptors for 5-methyltetrahydrofolate but allow the internal receptor pool to return to the cell surface. PMA does not affect the clustering of the folate receptor but instead markedly reduces the number of caveolae. Exposure to PMA totally blocks the intracellular accumulation of 5-methyltetrahydrofolate without affecting receptor-independent uptake or the formation of polyglutamylated species of 5-methyltetrahydrofolate in the cytoplasm. These data suggest that PMA inhibits uptake by inactivating caveolae internalization.