In this study, radiolabeled recombinant rat interleukin-1β (r<sup>125</sup>I-IL-1β) was used to localize and characterize IL-1β binding in rat hypothalamus and pituitary gland by quantitative autoradiography. The ability of this ligand to bind to type I IL· 1 receptor was first tested on murine lymphoma cells (EL-4). In the rat-tissue sections, high densities of specific r<sup>125</sup>I-IL-1β binding sites were localized in the anterior as well as the posterior pituitary and in the choroid plexus. A fine labeling was observed in meninges and third ventricle walls while no binding was detected in the hypothalamic nuclei. Saturation experiments, in the anterior and posterior pituitary, revealed one specific binding site with an affinity constant (K<sub>d</sub>) of 0.5 n M. Competition experiments were achieved using either rat IL-1β (rIL-1β) or human IL-1s (hIL-1α, hIL-1β and IL-1 receptor antagonist: hIL-1α). Affinity constants (Ki) were drastically different according to the ligand used, while K<sub>i</sub> values were found similar in anterior and posterior pituitary. Competition with rIL-1β revealed one binding affinity (K<sub>i</sub> of 0.1 n M range). In contrast, competition with hIL-1β revealed two binding affinities: a high (Ki: 0.1 pM range) and a low one (K<sub>i</sub>: 1 n M range). Competition with hlL-1ra was obtained for high concentrations only (K<sub>i</sub>: 10–100 n M range), whereas human IL-1α (hIL-1α) was unable to compete at 1-100 n M. Therefore, two types of IL·1 binding affinities have been identified in rat pituitary tissues: a very high affinity binding, detected with hIL-1β only and a lower affinity binding. Human IL-1β can therefore be a better ligand for rat receptors than the rat ligand itself while hIL·lα is not. This is suggestive for the existence of several IL-1 receptor subtypes and is in accordance with the biological activity of hIL-1β in the rat. Moreover, the high density of binding sites observed in both anterior and posterior pituitary indicates that pituitary tissues are direct targets for IL-1 stimulation. It is worth noting that IL-1 binding sites cannot be detected in the rat hypothalamus. This raises the question of the mechanism of the hypothalamic IL-1-mediated effects and suggests a role of the third ventricular walls in this mediation.