Plasma levels of prekallikrein, kallikrein, high molecular weight kininogen and kallikrein inhibitors were measured in samples from dogs infused with E. coli endotoxin. Markedly reduced levels of prekallikrein and high molecular weight kininogen were seen at circulatory collapse and free kallikrein was detected. Both ‘fast-reacting’ and ‘time-dependent’ inhibitors of plasma kallikrein were significantly reduced. Fractionation of pre-infusion and collapse samples by gel filtration together with immunological procedures indicated that both α2-macroglobulin and α1-antitrypsin were the major kallikrein inhibitors in dog plasma, and that they were significantly reduced in the collaps