Theoretical ab initio calculations have been performed to determine the stability of the covalent anions of adenine and the adenine-thymine (AT) Watson-Crick base pair. The conclusions of this work are as follows: (i) the covalent anion of adenine is predicted to be a stable system with respect to a vertical electron detachment, but unstable with respect to adiabatic detachment: (ii) the covalent anion of the adenine-thymine dimer (A-T-) has similar properties, although in this system the presence of the second base provides an additional stabilization to the excess electron: (iii) in A-T- the excess electron is localized at the thymine molecule, and this molecule＇s ring is puckered; (iv) no valence A-T anion was found in the calculations with the excess electron located at the adenine molecule: and (v) in view of the above results, we predict that the AT base pair is not an effective trap of excess electrons.