There is emerging evidence for a role of the CD40/CD40L dyad as a signalling mechanism in different inflammatory conditions. The aims of this study were to 1) quantify the constitutive and induced expression of CD40 in different regional vascular beds of the mouse, and 2) assess the role of CD40L as a modulator of vascular endothelial CD40 expression. The dual radiolabeled monoclonal antibody technique was used to quantify the expression of endothelial CD40 in control and lipopolysaccharide (LPS)-challenged wild-type (WT) mice. Significant constitutive CD40 expression was detected in several vascular beds of WT mice, with lung, kidney and small intestine exhibiting the highest expression, while the liver and stomach showed no detectable baseline expression. LPS administration elicited 2- to 7-fold increases in CD40 expression in several tissues (heart, kidney, intestine) within 4 h, while other organs (brain) required up to 48 h to exhibit CD40 upregulation. CD40 expression was not detected in unstimulated or LPS-challenged CD40^-/- mice. Constitutive expression of CD40 was profoundly reduced in unstimulated CD40L^-/- mice, but the LPS-induced CD40 upregulation did not differ between CD40L^-/- and WT mice. Depletion of platelets or T-lymphocytes, the major CD40L-expressing cells in blood, also resulted in a profound reduction in basal CD40 expression. These findings demonstrate significant endothelial expression of CD40 under basal conditions in different vascular beds and increased CD40 expression following endothelial cell activation with LPS. Platelet- and T-lymphocyte-associated CD40L appears to play a major role in regulating the density of CD40 expression on vascular endothelial cells in vivo.