In the light of the biological significance of thymine photodimers , studies of the energetics of the dodecanucleotide fragment d( CGCGAATTCGCG )2 have been carried out using the methods of molecular mechanics, with and without incorporation of a thymine dimer in the cis-syn configuration. The results of the calculations suggest that the thymine dimerized structures show no gross distortion in the double helix with the conformational changes relative to the normal B-DNA double helix restricted largely to the dimer region. The energetics of dTp[]dT reveal a number of conformers which are energetically almost equally favorable and are, as a group, qualitatively consistent with NMR studies on this molecule. The biological implications of the results of the conformational studies, reported here, have been examined vis-a-vis the currently available models for the recognition of DNA "damage" by repair enzymes.