The adhesion of human platelets to subendothelium of rabbit aorta and the formation of microthrombi (aggregation) were investigated in an annular chamber after perfusion of native or citrated blood at wall shear rates ranging from 650 to 3300 sec-1. Previous studies demonstrated reduced adhesion and increased thrombi with native as compared to citrated blood and suggested that thrombi grow larger and higher in native blood. In the present study, thrombus dimensions were quantified by newly developed stereological techniques. The thrombus volume per surface area of subendothelium increased with increasing shear rate and was significantly greater in native blood. At 1300 sec-1 thrombi less than 5 and less than 10 micrometers in height were more frequent in citrated blood, and those greater than 20 micrometers were much more frequent in native blood. Maximum thrombus heights were 62 +/- 7 and 71 +/- 5 micrometers in native and 17 +/- 2 and 28 +/- 4 in citrated blood at shear rates of 650 and 1300 sec-1, respectively. Surface coverage with fibrin was 30%, 11%, and 0% at shear rates of 650, 1300, and 3300 sec-1, respectively. In addition, the effects of approximately 15 and approximately 45 mM citrate (final concentration in plasma) were studied at shear rates of 200, 650, and 2600 sec-1. At a citrate concentration of approximately 45 mM aggregation was abolished, and adhesion was strongly inhibited at the highest shear rate. We conclude that citrate (1) inhibits platelet attachment kinetics, thus acting to reduce adhesion, and (2) inhibits platelet aggregation and thrombus stability, thus reducing thrombus heights, which (3) may indirectly lead to increased adhesion.