Electrophysiological recordings were made from 12 specific warm receptors in the sensory nerves of human hairy skin. In all cases examined, single warm fibers were spontaneously active at normal skin temperature from about 32 degrees C upwards, while touch, stretching, vibration or pricking the skin did not elicit any response within the receptive fields of warm units. Moderate warming of the skin using linear temperature rates from 0.5 degrees C/s to 1.5 degrees C/s caused a transient overshoot in the frequency of discharge with peak frequencies depending on the rate of change of temperature (dT/dt), the magnitude of thermal increment (deltaT) and the adapting temperature (TA). Cooling the skin caused a transient inhibition of discharge.