Single-neuron recordings were made from the lumbar spinal cords of cats and squirrel monkeys. Recording sites were distributed throughout the dorsal horn and included Rexed's laminae I and III-VI in both species and laminae VII-VIII in cats. Activity was studied during systematic changes in skin temperature over the range of 15-49 degress C; this encompasses the perceptions of innocuous cooling and warming plus the initial stages of noxious heating. The experiment included studies in which the thermal stimulus was changed from various preadapting temperatures. In all cases, the sensitivity of an individual neuron to changes in skin temperature was associated with responses to various intensities of tactile stimulation which, for some neurons, could range from low to painful pressures. More than two-thirds of the neurons excited by innocuous temperature changes discharged to both cooling and warming, although the thresholds were much lower for cold temperature differecnes (less than or equal to 2 degrees C for cold steps as compared with more than 6 degrees C for warm steps). However, many neurons only responded to extreme cooling or, more frequently, noxious heating. The temperature response relationships of many neurons during cooling was best described in reference to specific cold-receptor activity because the discharge rates declined at extremely cold temperatures and because the slopes of the temperature-response functions were nearly identical when studied with different adapting temperatures. The responses of certain slowly adapting mechanoreceptors was considered in describing some of the spinal cord activity during extreme cooling. The responses to hot temperatures were attributed to activity in various receptors, including especially polymodal receptors. Activity during innocuous warming was ascribed to one population of peripheral warm receptors that do not show maximal static activity during innocuous warm stimuli. The significance of the extensive convergence in the spinal cord from mechanoreceptors and thermoreceptors was discussed in relation to thermal perception and the complexity of the information transmitted by the spinothalamic tract.