This investigation was designed to determine the differences in tissue alteration produced by electrosurgical machines with different carrier frequencies and waveforms. Histologic analysis showed that: 1. The machine with full-wave rectification and the lowest frequency of operation (machine I, 1.7 MHz) produced significantly greater tissue alteration in the superficial tissue layers than the full-wave rectification machines with higher frequencies of operation. 2. The continuous-output waveform produced significantly less tissue alteration in the superficial tissue layers than the modulated type at the same frequency of operation. 3. Histologically, the experimental defect showed a band of coagulation necrosis approximately 50 mu wide along the margin of the incision. Tissue destruction decreased laterally and inferiorly from the surface of the defect. 4. The experimental defect was similar in shape to the physical form of the cutting electrode.