Vascular subendothelium and collagenous surfaces were exposed to flowing cltrated blood. Platelet interactions with these surfaces were Investigated at various axial distances from the upstream end of the exposed surfaces. A pronounced axial decrease In surface coverage with platelets and in thrombus dimensions was encountered on collagenous surfaces. This phenomenon was observed at shear rates of 200 to 2000 s~\ but was most pronounced at low shear rates (<650 s~1). After 5 minutes of perfusion at a shear rate of 650 s~\ 4.6x10* platelets were deposited on the most upstream 20 mm 2 of the collagen surface, In contrast to 2.2x10° platelets/ 20 mm 2 14 mm farther downstream. Depletion of von Wlllebrand factor and/or thrombospondin from the boundary layer of the blood flow was not responsible for this. Collagen-bound von Wlllebrand factor enhanced the surface coverage with platelets without affecting the axial decrement, while pretreatment of the collagen surface with thrombospondin had no effect at all. However, partial Inhibition of thrombus growth by aspirin reduced the axial decrements, and less thrombogenlc surfaces as human and rabbit subendothelium, which induced only a few small thrombi, produced virtually no axial differences In platelet adhesion. Raising the shear rate to 2600 s~1 also gave no axial differences In platelet-collagen adhesion; It did, however, give an axial Increase In thrombus dimensions. This Increase was neutralized after the addition of antibody against human platelet thrombospondin to the blood. Our data are consistent with the view that platelet-surface Interactions are limited by the arrival of platelets to the surface at shear rates below 650 s~1. Surfaces that Induce rapid-growing upstream thrombi may deplete the boundary layer for platelets, resulting In decreased platelet adhesion and thrombus growth farther downstream. At higher shear rates, when the platelet supply to the surface Is not a limiting factor, thrombospondin released from upstream thrombi appears to enhance downstream thrombus growth and/or thrombus stability. (Arteriosclerosis 9:33-42, January/February 1989)