Immobilization stress induced interleukin-1 beta (IL-1 beta) mRNA in the rat hypothalamus. IL-1 beta mRNA was induced at 30 min after the start of immobilization, reached a maximum at 60 min and then was still detected with a decreased level at 120 min. However, 240 min after the start of the immobilization, IL-1 beta mRNA became hardly detectable despite the continuance of immobilization. Distinct expression of IL-1 beta mRNA was not detected in any other brain region examined at 30 and 60 min after the start of immobilization. These results demonstrate that the immobilization stress induces the expression of IL-1 beta mRNA solely in the hypothalamus and suggest that IL-1 beta is involved in the response to stress there.