To understand the effects of ultrasound in biological systems at the molecular level, sonolysis of nucleic acid bases at sonic intensities .ltoreq. 5 W/cm2 was studied. These sonoreactions were followed by UV-spectral decrease and by sonoproduct analysis. The order of reactivity was thymine > uracil > cytosine > guanine > adenine. The extent of sonoreactions depends on the exposure time and the intensity rate. Aeration was necessary for maintenance of a reasonable reaction rate. Threshold intensities for uracil and thymine were observed at .apprx. 0.5 W/cm2. The effectiveness of the dissolved gases in producing sonoreactions was Ar > O2 > air > N2 > He > N2O, suggestive of free radicals mediating these reactions since N2O is an effective radical scavenger. Studies of the effects of substituents have shown that electronic rather than steric effects may have a greater influence. Preliminary identification indicates that cis- and trans-uracil glycols are the major products of uracil. A stepwise mechanism of pyrimidine sonolysis is proposed. Sonolysis of a dilute aqueous solution of uracil yielded pseudo-1st-order kinetics in terms of [Ura] with a rate constant of 3.9 .times. 10-2 min-1, implying that the rate-limiting step is the reaction of HO with the base.