Our laboratory has shown that peripheral inflammatory pain induced by lambda-carrageenan (CIP) can increase blood-brain barrier (BBB) permeability and alter tight junction (TJ) protein expression leading to changes in BBB functional integrity. However, the intracellular signaling mechanisms involved in this pathophysiologic response have not been elucidated. Transforming growth factor (TGF)-beta signaling pathways are known to regulate vascular integrity and permeability. Therefore, we examined the function of TGF-beta signaling at the BBB in rats subjected to CIP. During CIP, serum TGF-beta1 and protein expression of the TGF-beta receptor activin receptor-like kinase-5 (ALK5) were reduced. Brain permeability to (14)C-sucrose was increased and expression of TJ proteins (i.e., claudin-5, occludin, zonula occluden (ZO-1)) were also altered after 3 h CIP. Pharmacological inhibition of ALK5 with the selective inhibitor SB431542 further enhanced brain uptake of (14)C-sucrose, increased TJ protein expression (i.e., claudin-3, claudin-5, occludin, ZO-1), and decreased nuclear expression of TGF-beta/ALK5 signaling molecules (i.e., Smad2, Smad3), which suggests a role for TGF-beta/ALK5 signaling in the regulation of BBB integrity. Interestingly, administration of exogenous TGF-beta1 before CIP activated the TGF-beta/ALK5 pathway and reduced BBB permeability to (14)C-sucrose. Taken together, our data show that TGF-beta/ALK5 signaling is, in part, involved in the regulation of BBB functional integrity.