Agmatine, an amine and organic cation, reduced $H_2O_2$ that was generated by hyperglycemia, and transcription factors such as NF-${kappa}B$ and AP-1 activity in the mesangial cells that were exposed to high glucose. However, spermine which shares a strong nucleophilic structure with agmatine decreased the $H_2O_2$ levels and AP-1, but not the NF-${kappa}B$ activity. Possible roles for agmatine and spermine in decreasing fibronectin are discussed, and the signaling pathway for agmatine-reduced fibronectin accumulation is presented.