Certain physiologic and hematologic data were determined in ponies given Escherichia coli endotoxin by three routes: single IV dose, single intraperitoneal (IP) dose, and multiple IP boluses. In all ponies, the reaction was characterized by weakness, depression, peripheral circulatory abnormalities, and pyrexia. The pyrexia was more severe and was sustained in the ponies given multiple IP bolus endotoxin. Changes in packed cell volume, peripheral blood neutrophil, lymphocyte, and thrombocyte counts, and blood glucose were noticed in the three groups. Blood lactate and beta-glucuronidase values were determined and increases occurred only in the two IP endotoxin administration groups. A fibrinogen increase was observed in only the multiple IP bolus group. Attempts were made to correlate the lactate and beta-glucuronidase values with the severity and prognosis of the endotoxemia response. In general, the single IV bolus and, to a lesser extent, the single IP bolus endotoxin produced abrupt but transient responses. The multiple IP bolus endotoxin administration produced a more gradual and sustained response, which was more closely comparable with a clinical gastrointestinal disease problem than the other routes of administration produced.