Single cutaneous dorsal root fibers from receptive fields on the proximal-lateral portions of the cat's hind limb were isolated in anesthetized animals by the technique of dissection. Slowly adapting mechanoreceptors were studied in detail and their responses to systematic presentations of various physiological temperature levels (43-13 °C) were examined. The mechanoreceptors were initially grouped into type I and II receptors on the basis of receptive field characteristics, the gross morphology of the skin in the receptive areas, and the existence and/or regularity of spontaneous activity levels. This dichotomization was shown to be valid with respect to the dissimilarity in the sensitivity of the two receptor groups to thermal stimulation. Type II receptors were found to behave more like true, specific thermoreceptors because they were able to maintain different discharge rates for dissimilar constant temperature stimuli and were able to respond consistently and, with little variability, to phasic cooling over a wide range of temperature differences. Both groups of receptors were, however, shown to be capable of responding to rapid cooling. The response patterns to temperature from cat type II receptors were noted to be quite similar to bimodally sensitive primate receptors and the possibility of using this thermal sensitivity as a method for categorizing primate slowly adapting mechanoreceptors was indicated. The probable role of these bimodally sensitive receptors in thermal sensation was also discussed.