The levels of the recently isolated endothelial-derived peptide, endothelin, which has potent vasoconstrictor properties, were analyzed by radioimmunoassay in plasma from six patients with sepsis syndrome. For comparison endothelin levels were also measured in plasma from ten healthy volunteers. In the septic group plasma endothelin-like immunoreactivity was five-fold higher (11.3 +/- 2.8 pmol/l) compared to the volunteers (2.4 +/- 0.07 pmol/l) (P less than 0.01). Three patients had anuria and were found to have the highest plasma endothelin levels, maximally 23.8 pmol/l. In the septic group plasma endogenous norepinephrine was increased to 8.3 +/- 2.2 nmol/l, as compared to 0.98 +/- 0.09 nmol/l in healthy volunteers (P less than 0.01). The results confirm that the sympathetic nervous system is activated in the septic patient. Plasma levels of endothelin, probably emanating from damaged endothelial cells by bacterial toxins, are also elevated indicating that this peptide may be involved in vasoconstrictor responses resulting in organ failure, which so often is encountered in septicemia.