This study was undertaken to investigate the changes in feeding behavior, including ambulatory activity, induced by a single injection of Interleukin-1 beta (IL-1) (2 micrograms/rat) at 18:00, just before the dark phase. For this purpose, we used the Gunma University-type automatic apparatus for continuous and direct measurement of ambulation and drinking. A significant decrease in food intake was observed for 12 hours after treatment with IL-1. Peripheral administration of IL-1 also produced a marked decrease in ambulatory activity within 3 hours which continued for 6 hours. In addition, IL-1 produced a marked decrease in drinking behavior during the first 6 hours. We reported here the changes in consummatory and ambulatory behavior of rats after acute administration of IL-1. The sickness which IL-1 produced may, at least in part, contribute to these phenomena, although precise mechanisms are still unknown.