Pain and temperature stimuli activate neurons of lamina I within the dorsal horn of the spinal cord, and although these neurons can be classified into three basic morphological types and three major physiological classes, earlier studies did not establish a structure/function correlation between their morphology and their physiological responses. We recorded and intracellularly labeled 38 cat lamina I neurons. All 12 fusiform cells were nociceptive-specific, responsive only to pinch and/or heat. All 11 pyramidal cells were thermoreceptive-specific, responsive only to innocuous cooling. Of ten multipolar cells, six were polymodal, responsive to heat, pinch and cold, and four were nociceptive-specific. Five unclassified cells had features consistent with this pattern. These results support the view that central pain and temperature pathways contain anatomically discrete sets of modality-selective neurons.