Possible variations in the secondary structure and the A - B transition in DNA's of varying primary base sequence and composition have been studied by the techniques of X ray diffraction and molecular model building. The DNA's studied are from both bacteria and from eukaryotic cells. In addition a DNA of viral origin, from the bacteriophage OW-14, has been investigated. A computerised model building study of the changes induced in DNA secondary structure by the binding of intercalating drugs has also been carried out. A linked atom least squares routine has been extended and used to refine the models presented. The routine enables standard values for the parameters defining the covalent stereochemistry of the structure to be retained. Methods of calculating the Fourier transforms of the models produced are discussed, and this enables some comparisons to be made between the observed diffraction data and those predicted by the models. Structures studied include the intercalation complexes involving ethidium or daunomycin, general intercalation models for DNA and models for the conformation of the putrescene groups in OW-14 DNA.