Voltammetric, UV-vis spectroscopic and viscometric methods were used to investigate the interaction between norepinephrine and DNA. The results of square wave voltammetric and UV-absorption measurements have shown that norepinephrine was bound to DNA. Besides the electrostatic interaction between DNA and norepinephrine, hydrogen bonds and intercalation may also exist. Viscosity measurements also suggest that NE is bound to DNA via an electrostatic mode of interaction at lower norepinephrine concentrations. However, at higher concentrations of norpinephrine, the increase in the relative viscosity of DNA indicated that the intercalation is also involved in the binding. The binding constant and the binding site size for the interaction of norepinephrine with DNA were K = 3.3 (+/- 0.18) X 10(3)M(-1), s = 2.89 (+/- 0.05) base pairs and K = 5.15 (+/- 0.38) X 10(3)M(-1), s = 2.41 (+/- 0.04) base pairs for the limiting conditions of static and mobile binding equilibrium, respectively. The binding site size determined shows that norepinephrine covers more than 2 base pairs upon binding to DNA. The standard Gibbs free energy change (Delta G(0) = -RT ln K) comes out to be approximately -21.17 kJ/mol at 25 degrees C, which indicates the spontaneity of the binding of NE with DNA.