Concentrated solutions of low-molecular-weight DNA (Mw=35000) with a known molecular-weight distribution can be prepared in several hours, and require no additional fractionation procedures. This is achieved by sonication of the DNA in 1.0 M-NaCl at high power at 0--2 degrees C. No denaturation of the DNA is detectable, even after 8h of continuous sonication. After 2h, the molecular-weight distribution of the total DNA sample is that of the most probably Schulz distribution, described by-Mn:-Mw:-Mz ...=1:2:3 ...etc. Such a molecular-weight distribution is expected for a random break-up of indefinitely long macromolecules and indicates that the sonication process is essentially by random double-strand scission. DNA was also sonicated in the presence of ligands capable of modifying the DNA tertiary structure. The results support the idea that inflexibility of the DNA is required for efficient sonic degradation.