Thermal thresholds (cool, warm, heat, heat pain) were determined in four skin regions (cheek, glabrous skin of the hand, hairy forearm, leg) of eight healthy human subjects. The thermostimulator was composed of Peltier elements and three rates of continuous stimulation were used: 1.4, 2.4, and 3.9 degrees C/s. Warm, heat, and heat pain thresholds increased with increasing rate of temperature change, and the increase was of equal magnitude with these three thresholds. However, the effect of increasing stimulus rate on cool thresholds was nonsignificant. Similar results were obtained in all skin regions studied. It is suggested that liminal warm, heat, and heat pain sensations are mediated by afferent fibers with conduction velocities of the same range (C-fibers) whereas liminal cool sensations are signaled by faster conducting afferent fibers.