Previous work has shown that bacterial endotoxins activate an inducible form of histidine decarboxylase present in mammalian tissues. The product of this enzyme, induced histamine, has been postulated to have a circulatory function and to oppose the vasoconstrictor actions of the catecholamines on the small blood vessels. The present experiments show that in kidney of endotoxin-treated rabbits there is an anomalous response, histidine decarboxylase being strongly reduced, not activated. The findings are consistent with the view that suppression of induced histamine synthesis, at a time of markedly increased catecholamine release, might potentiate the actions of these vasoconstrictors on the kidney. This event may be a factor in the early endotoxin-induced changes in the glomerular capillaries which prepares them for the subsequent events of the generalized Shwartzman reaction.