Endotoxin shock (ES) was induced in Labrador retriever dogs by infusion of a lethal dose of Escherichia coli endotoxin. Spontaneous plasma kallikrein activity and prekallikrein levels were determined during subsequent stages of the shock both by an esterolytic assay (BAEe) and a new amidolytic assay utilizing a chromogenic tripeptide derivative (Chromozym PK, Pentapharm AG, Basel Switzerland). During the late stages of shock characterized by a substantial decline of blood pressure, plasma prekallikrein levels determined by the amidolytic assay were considerably reduced. In this phase of the shock both the esterolytic and the amidolytic assays revealed a significant elevation of spontaneous activity. Plasma prekallikrein determined by the esterolytic assay was found to be elevated vs. control values for the whole duration of ES. The findings made seem to associate activation of the plasma kallikrein-kinin system to the circulatory collapse of endotoxin shock in dogs.