This study was conducted to obtain information about the critical temperature of teh spinal cord in hyperthermia produced by radiofrequency waves allied to the spine. The first component of the spinal cord evoked potential was analyzed as an indicator of spinal cord function. The spinal cords were heated y radiofrequency waves to a maximym of 47°C momentarily of for 30 minutes. The temperatures were measured with a thermasensor in the epidural space. In momentary heating, the reductions in amplitude were almost parallel with the increases in temperature. In mainained heating for 30 minutes, at 44°C and below, the amplitudes decreased by one-quarter to three-quarters of the control value in the first 5 minutes and recovered to over three-quarters of the control value in 30 minutes. The amplitudes returned to almost the control value after restoration of normal spinal cord temperatures. At 45°C in the and above, however, the amplitudes were prominently reduced or disappeared in the first 5 minutes and remained depressed during the reemainder of the heating. On normalizing the temperature, the amplitudes did not return to the control value. These results suggest that 44°C in the epidural space is the highest tolerable temperature for normal spinal cord function.