1. The cardiovascular effects of intravenous injections of interleukin-1 (IL-1) and tumour necrosis factor (TNF) have been investigated in the conscious rabbit. They have been compared with the effects of bacterial lipopolysaccharide (LPS) because both IL-1 and TNF are released from macrophages by LPS.

2. IL-1, TNF and Escherichia coli J5-LPS all caused hypotension when given intravenously in a dose with low mortality. The time course of the hypotension caused by IL-1 and LPS was similar, although the maximal fall in mean blood pressure occurred earlier after IL-1. TNF produced a more sustained fall in blood pressure. Hypotension was not accompanied by a compensatory tachycardia after any of the test substances. Hypotension was associated with a fever after TNF, hypothermia after LPS and no significant change in temperature after IL-1.

3. The packed cell volume did not change during hypotension in any of the study groups, implying that the hypotension was not due to fluid loss resulting from increased capillary permeability.

4. IL-1 and TNF are candidates for the role of effectors of LPS-induced hypotension.

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