Dendrimers are highly branched macromolecules with the potential to be used for biomedical applications. Several dendrimers are toxic owing to their positively charged surfaces. However, this toxicity can be reduced by coating these peripheral cationic groups with carbohydrate residues. In this study, the toxicity of three types of 4th generation poly (propyleneimine) dendrimers were investigated in vivo; uncoated (PPI-g4) dendrimers, and dendrimers in which 25% or 100% of surface amino groups were coated with maltotriose (PPI-g4-25% m or PPI-g4-100% m), were administered to Wistar rats. Body weight, food and water consumption, and urine excretion were monitored daily. Blood was collected to investigate biochemical and hematological parameters, and the general condition and behavior of the animals were analyzed. Unmodified PPI dendrimers caused changes in the behavior of rats, a decrease in food and water consumption, and lower body weight gain. In the case of PPI-g4 and PPI-g4-25% m dendrimers, disturbances in urine and hematological and biochemical profiles returned to normal during the recovery period. PPI-g4-100% m was harmless to rats. The PPI dendrimers demonstrated dose-and sugar-modification-degree dependent toxicity. A higher dose of uncoated PPI dendrimers caused toxicity, but surface modification almost completely abolished this toxic effect. (C) 2011 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 99A: 261-268, 2011.