The aim of this study was to evaluate whether left ventricular hypertrophy in spontaneously hypertensive rats (SHR) is accompanied by myocardial collagen quantitative and/or qualitative changes. 15 SHR and 20 control normotensive rats (WKR) of three months of age were used. At this age, hypertension has already caused a significant increase in the ratio of the ventricular mass to body weight (mg/g) in hypertensive animals (SHR: 5.11 +/- 0.21; WKR: 3.40 +/- 0.22; p less than 0.001). With respect to body mass, the amount of collagen elicited from the hydroxyproline concentration increases in SHR but remains percentually the same with respect to the biventricular mass. In SHR, changes in the amount of type-1 alpha chains and type-V alpha chains, and the presence of a low molecular weight collagenous fraction have been observed. Moreover, we have found an increase in the ratio of type-1 alpha 1 chains to type-1 alpha 2 chains. This change might be related to the appearance of a type-1 alpha 1 trymer. The presence of such a type-1 alpha trymer and of low molecular weight collagenous fractions may suggest the appearance of fetal collagenous isoforms in ventricular myocardium, due to the increased pressure load as well as to the increased turnover (an index of a remodelling activity of cardiac stroma). These changes might play a role in the transformation of myocardial viscoelastic properties in SHR with a progressive diastolic stiffness of the ventricular wall.