Interleukin (IL-1) is a cytokine which plays an important role in the modulation of the acute response in host defense. This cytokine is also increased in patients with Alzheimer's disease. In the present experiment systemic injection of IL-1 beta (7.5-50 micrograms/kg) decreased extracellular acetylcholine in the hippocampus. This effect could not be attributed entirely to general malaise since lithium chloride (130 mg/kg) had the opposite effect. Heat-inactivation of the cytokine eliminated the reduction of extracellular ACh. The results give further evidence of a relationship between the immune system and the central nervous system and suggest a possible relationship between IL-1 and cholinergic function or dysfunction in the hippocampus.