Using fluorescein isothiocyanate-conjugated ovalbumin (OVA-FITC), 125I-mannan, or 125I-invertase as specific ligands for the mannose receptor, we have quantified its activity in mouse and rat hepatic sinusoidal endothelium (HSE), under both basal conditions and after lipopolysaccharide (LPS) or human recombinant interleukin-1β (IL-1β) stimulations. Mouse treatment for 4 hours with 5 μg/kg IL-1β significantly increased OVA-FITC uptake by HSE. Ligand uptake exhibited a sublobular compartmentalization: In control mice as well as in IL-1β-stimulated mice, the ligand distributed preferentially in the periportal and septal areas; no OVA-FITC was observed in the perivenous sinusoids. In vitro exposure of mouse HSE to 100 pg/mL LPS or 1 ng/mL IL-1β for 6 hours significantly (P < .01) increased OVA-FITC uptake. Blocking IL-1 receptors in HSE by addition of 100 ng/mL IL-1 receptor antagonist (IL-1Ra) before stimulation with LPS or IL-1β abrogated the increase in mannose receptor-mediated uptake. In vitro endocytosis assays showed that rat HSE uptake of 125I-mannan or 125I-invertase progressively increased with both exposure time and concentration of added IL-1β. Upregulation of mannose receptor-mediated uptake in response to IL-1β or LPS was also blocked by previous addition of IL-1Ra to rat HSE. Flow cytometric analysis showed a significant HSE heterogeneity in mannose receptor-mediated endocytosis in response to IL-1β treatment: type I endothelial cells (EC-I, defined by their small size and high cytoplasmic density) significantly (P < .01) increased OVA-FITC uptake compared with type II endothelial cells (EC-II, defined by their large size and low cytoplasmic density). In addition, the subset of EC-I contained three times more IL-1β-binding cells than the EC-II subset. Because EC-I and EC-II are preferentially located in the periportal and perivenous segments of hepatic sinusoids, respectively, these results suggest that IL-1β, apart from upregulating mannose receptor activity, contributes to the sublobular compartmentalization of this endothelial cell function.