The structure of vascular sinuses in rat bone marrow is modified as cells cross their wall. Endotoxin, by causing withdrawal of granulocytes from the circulation, induces a compensatory discharge of fresh granulocytes through the wall of sinuses. As a result the sinus wall becomes infiltrated by clusters of late-stage hematopoietic cells which appear in transmural passage. Two correlates of such infiltration are elevation of adventitial cells from the wall and the creation of adventitial spurs extending into perisinusal hematopoietic spaces. In untreated rats adventitial cells cover, on the average, about 65 per cent of the sinus wall. After 0.04 µg. endotoxin the cover is reduced, fitting a slope regressing at the rate of 0.22 per cent/min. (t = 3.1038, p = < 0.003), the last determinations made at 120 minutes. Values for the percentage of adventitial cover calculated from observations after endotoxin are 52.5 per cent in 90 minutes and 32 per cent in 120 minutes. Even after endotoxin, a number of sinuses retain full cover while a number of control sinuses, likely reflecting normal cell passages, have little adventitial cover. Endothelial cells form a continuous layer. Apertures occur in the wall normally and increase in number after endotoxin. These apertures may contain cells, apparently in passage, or may be free of cells. Lysosomal disruption occurs in mural cells, particularly in segments of wall associated with cellular passage and with gelatinous change.