The serum concentrations of amino-terminal procollagen type III and carboxy-terminal procollagen type I-derived peptides, which have been proposed as useful markers of the tissue synthesis of collagen types III and type I, respectively, were abnormally increased in patients with essential hypertension and became normal after angiotensin-converting enzyme (ACE) inhibition. An association was found between baseline serum concentrations of these peptides and left ventricular hypertrophy, diastolic dysfunction, and ventricular arrhythmias in hypertensive patients. On the other hand, increased serum concentration of the carboxy-terminal procollagen type I-derived peptide was found in spontaneously hypertensive rats compared with normotensive Wistar-Kyoto control rats. An association was found between the serum concentration of this peptide and the extent of myocardial fibrosis and the hydroxyproline concentration in the left ventricle of spontaneously hypertensive rats. It is proposed that procollagen-derived peptides in serum may be markers of exaggerated collagen tissue synthesis involved in hypertensive myocardial fibrosis.