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The aim of this study was to determine possible age-associated differences in human blood pressure regulation during an immunological challenge in healthy subjects.Eight healthy young volunteers (median age 24 yr) and nine healthy elderly volunteers (median age 66 yr) received an intravenous bolus injection of Eschericia coli endotoxin (2 ng kg–1). Blood pressure, heart rate and core temperature were monitored during the first 7 h. Plasma catecholamine concentrations were measured at hourly intervals.The elderly showed a significantly more pronounced decrease in mean arterial pressure 4–7 h after endotoxin administration compared with the young controls (ANOVA; age × time;P < 0.0005). This mainly reflected a decrease in the systolic blood pressure in the elderly. The heart rate of both groups increased without difference between groups. Increased plasma epinephrine concentrations were found 2–3 h after endotoxin administration in both groups. Five hours after the endotoxin challenge, the epinephrine concentration had returned to control values in the elderly group only, in spite of decreased blood pressure.In conclusion, healthy elderly subjects fail to maintain a constant mean arterial pressure in response to the immunological challenge of endotoxemia.