1. Extracellular recordings were made from 81 primate spinothalamic (STT) neurons in the L7-S1 segments of the spinal cord. The majority of the sample was recorded from within laminae IV-V. 2. The responses of STT neurons to noxious thermal stimulation of glabrous and hairy skin were studied in an attempt to identify a neural substrate for the differences in thermal sensation evoked by noxious stimulation of these two types of skin. In addition, the responses to graded mechanical stimuli were examined for evidence of differential sensitivity. 3. Thermal intensity-response functions were constructed from the alteration in the mean discharge rate produced by a 30-s thermal pulse of 43-55°C. Generally, the functions derived from stimulation of both hairy and glabrous skin were either linear or positively accelerating. Deceleration in the response functions was occasionally observed above 53°C. The population mean discharge rate derived from glabrous skin stimulation was significantly greater than that derived from hairy skin stimulation above 49°C. 4. Cluster analysis was used to assess whether the STT population could be partitioned into functionally relevant subgroups. No clustering was evident on the basis of the alteration in discharge rate during stimulation alone. Analysis of the alteration in mean discharge rate during and following thermal stimulation identified four groups; these were referred to as the A(mnr), B(mnr), C(mnr), and D(mnr) classes. The clustering was not dependent on differences in the responses evoked from hairy and glabrous skin. The mechanical and thermal sensitivities of each thermal class covaried. 5. The capacity of the STT population to code the quality of noxious thermal stimuli, as judged by changes in the across-neuron discharge pattern, was assessed with a multidimensional scaling technique (MDS). The results suggest that the population discharge could be used to order stimuli correctly from 45 to 55° C. Also, it was found that a substantial change in the population's discharge pattern occurred to stimuli between 47 and 49°C when delivered to hairy skin. A similar alteration in the population's discharge pattern occurred to glabrous skin stimuli near 51°C. These alterations in population behavior may underly the alterations in sensory quality in humans that occur in these temperature ranges when stimulating hairy and glabrous skin. 6. The possible roles of the thermally and mechanically based classes in thermal intensity and quality coding were examined. Within the lower intensity ranges (<49-51° C), the C(mnr) and D(mnr) classes appeared to be best suited to intensity coding. Above 49°C, the B(mnr) class also had a steep monotonic response function. On the basis of an MDS analysis, no strong role was found for any particular thermal class in quality coding of stimuli from 45 to 55° C. Because many of the afferent fibers coding noxious thermal stimuli are also mechanoreceptive, a similar analysis was performed on the type 2 and 3 classes. Both type 2 and 3 classes had monotonic intensity response functions, but the type 2 class had significantly larger mean responses above 47°C. An MDS analysis suggested that the type 2 class was capable of coding stimulus quality throughout the range tested, whereas the type 3 class was capable of distinguishing only a few temperature ranges.