Cell-penetrating peptides can deliver macromolecular cargo into cells and show promise as vectors for intracellular drug delivery. Internalization occurs predominantly via endocytosis, but the exact uptake mechanisms are not fully understood. We show quantitatively how penetratin, a 16-residue cationic peptide, stimulates fluid-phase endocytosis and triggers its own uptake into Chinese hamster ovarian cells, using a 70 kDa dextran to indicate macropinocytosis. The total cellular endocytotic rate is significantly less affected and we therefore propose up-regulation of macropinocytosis to occur at the expense of other types of endocytosis. By comparing penetratin to its analogs PenArg and PenLys, enriched in arginines and lysines, respectively, we show how these side-chains contribute to uptake efficiency. The degree of peptide and dextran uptake follows similar patterns regarding peptide concentration and arginine/lysine content (PenArg > penetratin > PenLys), indicating that a high content of arginines is beneficial but not necessary for stimulating endocytosis. (C) 2008 Elsevier Inc. All rights reserved.