1. An extracellular study of the cat medial thalamus has revealed four types of somatosensory neurons. These were located primarily in the n. parafascicularis, n. subparafascicularis, and n. centralis lateralis; none were found in the n. centrum medianum. There was no functional segregation of neurons within each nucleus or between nuclei. Each type of neuron had large and often bilateral receptive areas. No somatotopic organization of neurons was found within the medial thalamus. 2. Noxious (N) and noxious-tap (NT) neurons comprising 72% of the sample (78 of 109 total) were considered to be nociceptive. N cells responded exclusively to noxious mechanical stimulation of skin, muscle fascia, tendons, and joints, and to direct stimulation of A-delta- and C-fiber groups in cutaneous, articular, and muscle nerves. NT cells responded to noxious and tap stimulation in a differential manner and to stimulation of the entire spectrum of A- and C-fibers. N and NT cells accurately signaled the duration of noxious mechanical stimulation. Their nociceptive responses were also graded as a function of both noxious stimulus intensity and the number of activated A-delta- and C-fibers. Stimulation of A- and C-fibers evoked, respectively, an inital burst and a late burst of discharges. A brief period of inhibition intervened between the initial and late bursts of NT cells. Prolonged afterdischarge was often observed following noxious natural stimulation or stimulation of A-delta- and C-fibers. The phenomenon of discharge "windup" was observed during iterative stimulation of C-fibers. 3. Tap (T) neurons (10%) responded only to brisk but innocuous taps applied to skin or underlying tissue. These cells were driven only by activation of A-alpha- and A-beta-fibers. The response to such stimulation was seen as an initial burst of discharges followed by an inhibitory period. 4. Inhibited (I) neurons (18%) had resting discharges that were inhibited by noxious stimuli and stimulation of A-beta- and C-fiber groups. 5. The results obtained from monitoring the peripherally evoked responses of nociceptive N and NT neurons before and after selective lesions of the spinal cord strongly suggested that the spinothalamic tracts were the only spinal projections mediating A- and C-fiber input to these cells. Each spinothalamic tract apparently carried information originating from both sides of the body.