Carrageenan injected into the rat pleural cavity induces after a short delay extravasation of plasma protein, the appearance of a neutrophil chemotactic activity (CA) and, by 4 hr, the infiltration of large numbers of neutrophils (80-140 x 10 the sixth cells/cavity). Indomehtacin (I) given intravenously suppressed these reactions primarily by suppressing protein exudation; both total protein and CA were reduced equally but the specific activity (CA/mg protein) was unchanged. In contrast to carrageenan, the Dextrans (T10, 40, 70, 150, 500, 2000) provoked upon injection mast cell degranulation and rapid accumulation of watery exudate with little protein and a few leukocytes. This reaction was not suppressed by I. The studies indicated that extravasation of water and protein may occur by different mechanisms and that, in the examples studied, leucocytaxia was associated with protein leakage.