Cold units innervating the cat's face were studied by extracellular recording in the trigeminal ganglion. The responses of the units to 2.5° C cooling at six rates (0.04, 0.06, 0.1, 0.5, 1, and 2° C/s) from three adapting temperatures (20, 30, and 40° C) were examined. The dynamic responses of the units were examined by calculating peristimulus time histograms and the average frequency during a period including the temperature change and the first 3 seconds at the lower temperature (referred to as the dynamic frequency). These measures were averaged across the units. Varying the adapting temperature produced a change in the magnitude of the units' responses to the cool stimuli, with the largest responses occuring at 30° C. At all adapting temperatures, cooling at rates of 0.5° C/s and above produced a rapid increase in firing frequency to a peak, and a less rapid decrease in frequency to a new steady-state level. The sharp response peak was absent for cooling at rates of 0.1° C/s and less. The effect of cooling rate on the peristimulus time histogram peak frequencies and the average dynamic frequencies was compared with human psychophysical data. The average dynamic frequency, rather than the magnitude of the response peak, correlates best with human sensitivity.