Thermal factors are believed to play a dominant role in the development of the structural and functional effects of irradiation of the nervous system with focused ultrasound at low‐megahertz frequencies. Similar mechanisms are postulated to underlie the effects of irradiation in methacrylate, which is frequently used as a test material to evaluate the influence of various factors on the results obtained. This study was undertaken to determine if thermal mechanisms alone can explain the development of trackless focal alterations (lesions) and all of their measurable characteristics in plastic as well as in brain. A purely thermal model is assumed and analytical prediction of lesion development and lesion size and shape for varying values of ultrasonic and thermal constants and controllable variables (frequency, focusing, dosage, target depth, etc.) is attempted. An empirical equation to describe the axial and radial ultrasonic energy distribution at the focus in water is derived. Appropriate heat transfer equations are developed for temperature distributions resulting from ultrasonic irradiation. The computed temperature profiles are plotted against nondimensionalized parameters. Temperatures at the lesion boundary were determined experimentally. Lesion dimensions read off the computed temperature profiles at the measured lesion boundary temperature are compared with experimental data. Agreement of analysis and data shows that, within the range of ultrasonic parameters used in this study, the development of lesions in the brain are explained by thermal mechanisms.