Polyamidoamine (PAMAM) dendrimers are cationic, branch-like spherical polymers being investigated for a variety of applications in nanomedicine including nucleic acid drug delivery. However, little is known about their toxicology in terms of their interactions with signal transduction pathways both in vitro and in vivo. We previously showed that a commercially available fragmented (or activated) generation (G) 6 PAMAM dendrimer, Superfect (SF), stimulated epidermal growth factor receptor (EGFR) tyrosine kinase signaling – an important signaling cascade that regulates cell growth, differentiation, migration, survival and apoptosis- in cultured human embryonic kidney (HEK-293) cells (Akhtar et al, 2013). Here, we firstly studied the in vitro effects of Polyfect (PF), an intact G6 PAMAM dendrimer, on EGFR tyrosine kinase signaling via extracellular-regulated kinase 1/2 (ERK1/2) in cultured HEK-293 cells and then compared the in vivo effects of a single i.p. administration of PF or SF (at 10mg/kg) on EGFR signaling in the kidneys of normal and diabetic male Wistar rats. Diabetes was induced by a single 55mg/kg i.p injection of streptozotocin and animals were sacrificed by cervical dislocation after 4 weeks. Dendrimers were administered 24 h before sacrifice. In cultured human embryonic kidney (HEK-293) cells, Polyfect, exhibited a dose- (0.4 to 40 microgram/mL) and time (1h to 24h) -dependent inhibition of EGFR phosphorylation in a manner similar to that observed for AG1478, a selective EGFR tyrosine kinase inhibitor. For example, treatment of cells with PF at 40 microgram/mL dose for 24h significantly reduced EGFR phosphorylation to 12.5 + 2.1% of vehicle-treated controls (p>0.05; N=6; Mean + s.d. shown; typically mean values were compared using analysis of variance followed by post hoc test (Bonferroni)). Administration of PF or SF to non-diabetic or diabetic animals (N=4-6) with subsequent study of EGFR phosphorylation in rat kidneys 24 h post-administration by Western blotting showed that PF inhibited EGFR phosphorylation by about 40% in both sets of animals whereas SF stimulated phosphorylation by 239 + 13% relative to control in non-diabetic and 141 + 10% in diabetic animals. Interestingly, the opposing effects of SF and PF on EGFR phosphorylation could be significantly reversed by the antioxidants apocynin (100 mM), catalase (2000 units/ml) or tempol (1 mM) in HEK-293 cells implying that both the stimulatory and inhibitory effects involved an oxidative stress dependent mechanism. These results show for the first time that PAMAM dendrimers can modulate the important EGFR-ERK1/2 cellular signal transduction pathway both in vitro and in vivo and may have important clinical implications for the safe use of these polymers in nanomedicine.