Single unit activity was recorded extra- and intracellularly in the thalamus of the cat following electrical stimulation of tooth pulp afferents. Cells activated by noxious stimuli were found in the basal ventromedial nucleus (VMB), the marginal zones of the arcuate and external nuclei of the ventrobasal complex (VBA and VBX) and the intralaminar complex. Cells found in any of these locations showed a variety of properties; they were activated at different latencies and had different patterns of input. However, cells with responses of short latency and low convergence were with few exceptions found in or close to VMB. Stimulation of tooth pulp evoked both EPSP's and IPSP's in these cells. A subpopulation of cells were found that responded to stimulation of either right or left tooth pulp afferents. 22 units, mainly located in VMB, were found that responded exclusively to stimulation of tooth pulp afferents. In the laterodorsal part of VMB cells influenced by tooth pulp stimulation were found to be antidromically activated by stimulation of the cortical tooth pulp projection area. No such cells were seen at other locations. The results of this study is in agreement with the conclusions in the companion paper (Rydenhag et al. 1986a) that VMB and the border zone between VMB and VBA are the most likely relay nuclei between tooth pulp nociceptive afferents and the cortex. The intralaminar complex is suggested to be important in the cortical arousal reactions and direction of attention following a painful stimulus.