Openings in the endothelium of frog mesenteric microvessels associated with acute inflammation following mild thermal injury have been examined by reconstruction from serial ultrathin sections. While all the openings lay close to the intercellular junctions only a minority (seven out of thirty-eight) were continuous with the intercellular clefts. The majority of gaps or openings (thirty-one out of thirty-eight) were transcellular and passed through the peripheral cytoplasm of one endothelial cell, separated from the intercellular cleft by a cellular process which was usually less than 2 microns wide.