Endothelin-1(ET) is a recently discovered endothelium-derived vasoconstrictor peptide. Plasma concentrations of immunoreactive (IR-) endothelin were investigated in rats by radioimmunoassay after extraction of endothelin on Sep-Pak Cartridges. In control rats, a mean concentration of 1.03 +/- 0.2 pg/ml was found. After exercise performed until semi-exhaustion or exhaustion, the mean plasma concentration of IR-endothelin decreased. An increase of Atrial Natriuretic Peptide plasma was observed immediately after exercise. There was a negative correlation between Endothelin and Atrial Natriuretic Peptide (r = -0.73, p less than 0.001). This is the first time that this increase following exercise is shown in normotensive rat. The mechanisms of the decrease of Endothelin are unclear. One hypothesis could be that the decrease in Endothelin is related to its rapid removal by the different tissues, specially the lungs.