The microcirculation in a transplantable rat fibrosarcoma was investigated. The vasculature of the neoplasm consisted of irregular channels lined by plump endothelium which displayed mainly pentalaminar although a few heptalaminar junctions were also found. Few pericytes surround the endothelial layer while the basement-membrane varied in thickness and was often duplicated and triplicated. Challenge with histamine resulted in increased permeability in comparison with normal connective tissues similarly treated. The increased permeability was accompanied by the formation of interendothelial gaps in these irregular vascular channels. Carrageenan induced a leucocytic exudate in the neoplasm which, apart from an increase in the mononuclear concentration 2 wk after injection, varied little from normal connective tissues. It is concluded that the majority of vascular channels in this neoplasm bear some structural and functional relationship to small venules.