Despite the frequent clinical use of ultrasound (US), neither its effect on nerve conduction nor the explanation for these changes have been agreed upon. This study differentiated between the thermal and mechanical effects of US by using continuous US to provide combined mechanical and heating effects, pulsed US to provide an equivalent mechanical effect, placebo US to duplicate the pulsed US treatment, and infrared radiation to provide a heating effect only. Ten subjects were randomly assigned to each of four treatment groups and the distal humeral segment of the ulnar nerve was treated. Statistically significant changes in both nerve conduction velocity (NCV) and subcutaneous tissue temperature were associated with each treatment. Continuous US and infrared radiation treatments were associated with increased temperatures (0.8C) and increased velocities (3.75 and 3.08m/sec, respectively). Pulsed and placebo US treatments were associated with decreased temperatures (2.2C and 3.1C, respectively) and decreased velocities (2.79 and 5.38m/sec, respectively). Similar levels and patterns for NCV and subcutaneous tissue temperature were observed for the continuous US and the infrared groups, and for the pulsed US and the placebo US groups. Additionally, placebo US and infrared radiation treatments produced opposite temperature and velocity changes, while not contributing any mechanical effects. It was concluded that the mechanical effects of US were not significantly operative in this study. The increased velocities associated with continuous US and infrared radiation treatments were attributed to a thermal-heating effect, and the decreased velocities associated with placebo and pulsed US treatments were attributed to a thermal-cooling effect of the US transmission gel.