Many thermosensitive neurons in the preoptic area-anterior hypothalamus (POAH) are believed to function in thermoregulation. Although many other diencephalic regions are implicated in thermoregulation, measurements of single-cell activity during localized thermal stimulation in these regions are lacking. Utilizing horizontal tissue slices, we have recorded single-unit activity throughout the rat diencephalon in response to localized thermal stimulation. Thermosensitive cells were identified in 18 nuclei. The proportions of each subpopulation inside vs. outside the POAH were similar: POAH (n = 83 cells); warm = 31%, cold = 4%, warm-cold = 1%, and temperature insensitive = 64%, outside POAH (n = 198 cells; warm = 39%, cold = 6%, warm-cold = 4%, and temperature insensitive = 51%. However, nuclei located rostral and lateral to POAH contained a large percentage of warm-sensitive cells (49-63%). Caudal nuclei contained approximately half of the cold-sensitive cells studied. This wide distribution of thermo-sensitive cells suggests that many diencephalic areas, besides the POAH, are capable of thermoreception and thermointegration. Moreover, many of these thermosensitive cells may function in other systems (e.g., reproduction, feeding, and water balance) which central and environmental temperatures are known to influence.