In the following experiments we attempt to quantitate the range of plasma catecholamine levels during canine endotoxin shock and to clarify some of the mechanisms involved in the changes. Basic to these investigations was a sensitive and reliable method for recovering the amines in plasma, a need that had been reflected in other studies (1-4).The availability of such an assay permitted a series of comparative studies on normal anesthetized animals, on those given varying doses of endotoxin at different rates of administration, on dogs subjected to adrenalectomy and high cervical cord section and then given endotoxin, and on the effects of endotoxin on catecholamine levels in endotoxin-resistant dogs.It was observed that the plasma concentrations of epinephrine were related to the level of the systemic arterial blood pressure although other factors also appeared to affect the amounts. Changes in blood pressure were not correlated with variations in the plasma content of norepinephrine. Repeated challenge with large doses of endotoxin conferred resistance to its lethal action, but this effect was not associated with an alteration in the pattern of the plasma catecholamine response. After cervical cord section shock could be induced in the absence of detectable plasma catecholamine, and under these conditions shock was significantly accelerated.* Submitted for publication June 25, 1965; accepted September 30, 1965. Supported by U. S. Public Health Service research grant AI-04415-04.