Interleukin-1 receptor type I and interleukin-1 receptor antagonist were found in magnocellular neurons of the paraventricular and supraoptic nuclei of the rat hypothalamus by immunohistochemical detection. Double-labelling experiments revealed that both proteins occurred in vasopressin-containing neurons. A similar distribution pattern was observed in a group of vasopressin-positive accessory magnocellular neurons. Axons emanating from the interleukin-1 receptor type I- and interleukin-1 receptor antagonist- immunoreactive neuronal cell bodies could be seen within the hypothalamic nuclei, and varicosities expressing interleukin-1 receptor antagonist immunoreactivity were observed in the internal zone of the median eminence, as well as in the hypothalamo-pituitary projection. The co-localization of interleukin-1 receptor type I with vasopressin is in agreement with findings that interleukin-1 has a stimulatory effect on vasopressin synthesis and release. The hypothalamic neurons may serve as a source of interleukin-1 receptor antagonist to balance the effects of interleukin-1.