The modification of glassy carbon and gold electrodes with DNA by adsorption or covalent immobilization in a mono- or submonolayer has been investigated using the couple Co(bpy)(3)(3+/2+) as an indicator. It has been found that when the solution containing double stranded or single stranded DNA is evaporated to dryness, dehydrated DNA molecules can be irreversibly adsorbed on the surfaces of glassy carbon electrodes, in an amount close to that of the saturated adsorptive monolayer. The DNA-adsorbed layer on glassy carbon electrodes is unstable to bases, but stable to 1 M HCl solution. The adsorption of DNA on the electrodes can be evaluated from the increase in the peak current, the decrease in the value of Delta E(p), and the negative shift in the value of E(o’) for the Co(bpy)(3)(3+/2+) couple. DNA is very strongly adsorbed on the oxidized surfaces of glassy carbon electrodes, and the adsorptive layer is very stable towards heating. The covalent immobilization of DNA directly onto the electrode surfaces is impossible due to considerable steric hindrance; but if the active groups (sites) on the electrode surfaces are elongated with other suitable molecules, the covalent immobilization of DNA becomes possible on the electrode surfaces. The quantity of covalently immobilized DNA at the electrodes reported in the paper is about 31% of the saturated monolayer.