Proposed mechanism for TSP1 antagonism of NO/cGMP signaling in platelets. Using recombinant domains and peptides of TSP1, we show that ligation of CD36 or CD47 is sufficient to block an NO-mediated delay in platelet aggregation. TSP1 blocks a delay mediated by either exogenous NO or NO synthesized by endogenous eNOS using Arg as substrate. The ability of TSP1 to prevent cGMP synthesis stimulated by exogenous NO identifies sGC as one target of TSP1 signaling. The ability of TSP1 to inhibit cGK-I–mediated phosphorylation of VASP and a cGK-I–selective peptide (RKRSRAE) stimulated by a cell-permeable cGMP analog (8Br-cGMP) identifies cGK-I as a second target of TSP1 signaling in platelets. VASP is required for NO/cGMP-mediated inhibition of agonist-induced platelet aggregation⁴⁴  as well as platelet adhesion.⁵⁵  TSP1 prevents cGK-I–mediated phosphorylation of VASP at Ser²³⁹. NO also stimulates phosphorylation of the cGK-I target Rap1GAP2,⁴⁰  so TSP1 inhibition of sGC and cGK-I also controls GTP loading of Rap1, which is required for thrombin-stimulated activation of the adhesion receptor α_IIb/β₃.