The present study examined changes in left ventricular design in spontaneously hypertensive (SHR) and normotensive Wistar-Kyoto (WKY) rats in response to different types of antihypertensive therapy. Blood pressure was reduced for 12 weeks by either peripheral vasodilators [hydralazine or felodipine (calcium antagonist)] or by sympatholytic drugs (alpha-methyldopa or metoprolol). End diastolic volumes (EDV) were obtained in vitro by determining the diastolic pressure-volume relationships of isolated perfused hearts, arrested in diastole by excluding calcium from the perfusate. Wall thickness (w) and internal radius (ri) were calculated from a ventricle considered as a spherical model. Compared with WKY, untreated SHR had elevated EDV and w. Vasodilator therapy, particularly felodipine, increased EDV but reduced w, while sympatholytic therapy with alpha-methyldopa reduced EDV in SHR. It is suggested that cardiac design is affected not only by the prevailing arterial pressure level which will affect w, but also by the haemodynamic situation. Vasodilators turn such a situation into one characterized by increased cardiac output and hence increased cardiac filling, whereas sympatholytic drugs by venodilatation will turn the haemodynamic situation towards a state characterized by reduced cardiac filling. Left ventricular EDV (ri) therefore seems to adapt to long-term filling conditions, while w adapts to bring w:ri ratio in balance with the pressure load.