The prolongation of the bleeding time by aspirin is presumably due to interfering with platelet function. Direct quantitative studies evaluating the effects of aspirin on the platelet component of the hemostatic plug have not been described. We measured blood loss from a standard ear injury in rabbits after treatment with either 5 mg or 200 mg/kg of aspirin (ASA) or sodium salicylate (SA), and related this observation to the number of platelets incorporated into the hemostatic plug. The high dose of aspirin was chosen since this dose inhibits PGI2 biosynthesis. Both doses of aspirin but not salicylate caused a significant increase on blood loss from the treated ear compared to the control (ASA 5 mg/kg, 0.012 +/- 0.009 ml, (m +/- SE), n = 44, p = 0.03; ASA 200 mg/kg, 0.02 +/- 0.007 ml, n = 17, p less than 0.05). Both doses of aspirin also caused a significant increase in the number of platelets incorporated into the hemostatic plug when compared to the SA treated animals (ASA 5 mg/kg, 3.52 +/- 0.34 X 10(6) platelets per incision, (m +/- SE), n = 59; SA 5 mg/kg, 1.9 +/- 0.15 X 10(6) platelets per incision, n = 54, p less than 0.001; ASA 200 mg/kg, 3.19 +/- 0.54 X 10(6) platelets per incision, n = 22; SA 200 mg/kg, 1.5 +/- 0.26 X 10(6) platelets per incision, n = 23, p less than 0.001). This study suggests that following aspirin administration hemostasis is achieved by the incorporation of a greater number of platelets into the platelet plug.(ABSTRACT TRUNCATED AT 250 WORDS)