Endothelial cell activation by thrombin is a key event in wound healing, Inflammation, and hemostasis. To better define thrombin‐endothelial cell interactions we synthesized several peptides of varying length corresponding to the initial 14 amino acid sequence of the cloned human platelet thrombin receptor after cleavage at an arginine⁴¹ site (R/SFLLRNPNDKYEPF). Thrombin receptor activating peptides (TRAPs) as short as 5 amino acids induced significant levels of PGl₂ synthesis and expression of PDGF mRNA in human endothelium and produced dose‐dependent cellular contraction and permeability of confluent human umbilical vein and bovine pulmonary artery endothelial monolayers. To explore whether TRAPs utilized similar signal transducing pathways as α‐thrombin to accomplish endothelial cell activation, phospholipase C production of the Ca²⁺ secretagogue IP₃ was measured and detected 10 seconds after either TRAP 7 or α‐thrombin. Furthermore, TRAPs ranging from 5‐14 residues induced significant dose‐dependent incsreases in Fura‐2 fluorescence indicative of Ca²⁺ ₁ mobilization. These results indicate that thrombin‐mediated proteolytic cleavage of the human and bovine thrombin receptor initiates stimulus/coupling respones such phospholipase C activation, Ca²⁺ mobilization, and protein kinase C activation. The functional consequence of this cellular activation via the cleaved receptor is enhanced cellular contraction, barrier dysfunction, PGI₂ synthesis, and expression of PDGF mRNA. © 1993 Wiley‐Liss, Inc.