The presence and localization of interleukin 1β immunoreactivity (IL 1β i.r.) was studied in the hypothalamus of four healthy, male pigs at 7 months of age, using immunocytochemical techniques on 100 μ vibratome and 10 μ paraffine sections. IL 1β i.r. was found in neuronal cell bodies and their processes within nuclei and fiber of the fiber tracts of the hyphotalamus as well as in varicose fibers, terminals and deposits within the median eminence. In addition, IL 1β i.r. was found in the walls of several, but not all, blood vessels and in very few glial cells.