We quantified the timing and extent of leukocyte retention by the coronary microcirculation in a pig model of hyperdynamic sepsis in three ways. First, the transmyocardial leukocyte gradient was determined as coronary blood flow (calibrated ultrasonic flow probe) multiplied by the difference between leukocyte counts in the aorta and coronary sinus. Measurements were taken at 1-min intervals for 30 min and then at 3-min intervals for 45 min in anesthetized pigs exposed to either endotoxin (50 micrograms/kg iv over 30 min) (n = 7) or vehicle (n = 7). Second, postmortem morphometric analysis was used to quantitate the number and location of retained myocardial leukocytes. Finally, myocardial capillary transit time of leukocytes was calculated from the above measures. In the endotoxin group 2.1 +/- 0.8 x 10(9) leukocytes/100 g wet wt were retained in the coronary circulation, primarily in capillaries. This resulted in 111 +/- 37 (P < 0.05) times as many leukocytes in the coronary microcirculation than predicted from the arterial leukocyte concentration. Myocardial capillary transit time of leukocytes was prolonged to 39.1 +/- 20.6 s (P < 0.05) in the endotoxin group versus 5.0 +/- 1.4 s in the control group. We conclude that, after endotoxin infusion in a pig model of hyperdynamic sepsis, myocardial leukocyte transit is slowed, leading to the retention of large numbers of leukocytes in the coronary microcirculation.