Photochemical damage to DNA induced by ultraviolet light is repaired in vivo by remarkable enzymes that are themselves dependent on light to carry out their catalytic role. The substrate for the repair reaction is a pyrimidine dimer (cyclobutadipyrimidine). The use of visible light to effect repair apparently has at its basis a photoinitiated electron transfer from an enzyme-bound photosensitizer to enzyme-bound, dimer-containing DNA. To study the intramolecular electron transfer reaction and the photochemistry of the dimer splitting process, we have devised, syntheisized, and studied a variety of compounds in which dimers are covalently linked to sensitizers. The results of such studies have cast light on the mechanistic features of enzyme- catalyzed repair of pyrimidine dimers in DNA.