Endothelin-1,−2, and−3 (ET-1,−2,−3) and their precursors, the big endothelins (big ET) are a family of novel endothelium derived vasoconstrictor peptides. Concentrations varying between 0.1–48 fmol·ml−1have been reported for ET-1 in the plasma of normal human subjects whereas little is known as yet about ET-3 and big ET concentrations in plasma. We have evaluated and optimized the radioimmunological determination of human ET-1, ET-3 and big ET-1 with respect to the mode of extraction of these peptides from plasma, buffer conditions and non-ionic detergents. It was shown that each of these factors could adversely affect the amounts of immunoreactive ET-1 detected in plasma, whereas immunoreactive ET-3 and big ET-1 concentrations were less influenced by these assay conditions. For the first time, the concentrations of immunoreactive ET-1 determined in plasma after silica gel extraction were confirmed by a radio receptor assay and by specific immunoextraction. The wide variety of ET-1 concentrations reported so far for plasma from normal human subjects can be explained to a high extend by these findings.