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