Short circulation is a disadvantage of dendrimers as drug delivery carriers due to their small size. In this work, pH-sensitive morpholino-terminated generation 5 poly(amido amine) (PAMAM) is prepared. At neutral pH, the dendrimer has a hydrophilic neutral surface and thereby stealth property. Under tumor acidic environments, the dendrimer develops positive charges due to protonation of the morpholino groups, and cellular uptake of the dendrimer is subsequently enhanced. The dendrimer is predicted to penetrate deeper within the tumor due to its small size (similar to 9 nm), and thereby the pH-sensitive function can be efficiently achieved because the microenvironment in the deeper sites is more acidic. Moreover, the morpholino-terminated PAMAM exhibited longer in vivo circulation time compared to PAMAM and hydroxyl-terminated PAMAM, partly compensating for the disadvantage of short circulation of dendrimers. Furthermore, the morpholino-terminated PAMAM also showed lower cytotoxicity than PAMAM and hydroxyl-terminated PAMAM. (C) 2016 Elsevier Ltd. All rights reserved.