Discharges were recorded extracellularly with tungsten microelectrodes from single neurons that were located in the cat's caudal bulbar reticular formation in anesthetized or decerebrated preparations. The responses to qualitatively and quantitatively graded mechanical and thermal stimuli were analyzed with respect to the lowest stimulus intensity needed to elicit a response, the response pattern evoked by the adequate stimulus, the receptive field size and the position of the receptive field on the body surface. The most typical neuron found in both types of preparations had an excitatory response when a heavy-noxious somatic stimulus was applied to a receptive field that was restricted to the face. The cutaneous fields were large and in some cases extended over the entire head. The qualitative classification of a heavy-noxious stimulus was confirmed by a close correlation between thermal and pressure thresholds of the units and psychophysical evaluations of various stimulus intensities. It was concluded that the caudal medullary reticular formation was physiologically distinguishable from more rostral parts of the brain stem reticular formation because of the functional specificity of a large proportion of the units in this part of the reticular formation.