In an effort to understand the conformational and structural changes in DNA brought about by thymine photodimer, computer modeling and molecular mechanics energy calculations were performed on DNA hexamer and dodecamer duplexes containing a cis-syn photodimer. The conformation of the crystal structure of the cyanoethyl phosphate ester of the thymine dimer (Hruska et al., Biopolymers 25, 1399-1417 (1986)) was used in modeling the photodimer portion. Various starting conformations were used in the modeling procedure and the structures were minimized both retaining and later relaxing the crystallographic geometry of the cyclobutane ring. The results indicate that most of the deformation is restricted to the thymine dimer region, and that the conformational changes decrease rapidly on either side of the region containing the photodimer. The structural changes brought about by the introduction of the photodimer can be accommodated within six base paired duplex without significant bend in the DNA. More conformational changes are observed on the 5'-side of the photodimer than on the 3'-side. The conformational features, such as backbone torsion angles and sugar puckers, of the energy minimized structures are discussed in the context of the solution structures determined by NMR on a series of oligomers containing photodimers (Rycyna et al., Biochemistry 27, 3152-3163 (1988)).