According to Maxwell's model, left ventricular (LV) systolic elastic properties are characterized by a series-elastic element and LV diastolic properties by a parallel-elastic element. It is not known whether this functional concept is reflected by some morphological counterparts. In 12 patients with aortic stenosis (AS) before and 19 months after aortic valve replacement, and in ten controls, LV biplane volume and simultaneous high-fidelity pressure measurements were performed during diagnostic catheterization. The constant (k) of systolic myocardial stiffness was determined from LV peak wall stress, the maximum rate of systolic stress rise and instantaneous LV midwall circumferential fiber shortening rate at peak stress. The constant (k*; circ-1) of diastolic stiffness was assessed from the diastolic stress-stiffness relationship, using a viscoelastic model. In AS patients pre- and postoperative endomyocardial biopsies were taken to assess muscle fiber diameter, interstitial fibrosis and LV fibrous content. Systolic myocardial stiffness constant was 15 circ-1 in controls, 14 circ-1 in preoperative and 12 circ-1 in postoperative AS patients (P less than 0.01 vs. controls). Diastolic myocardial stiffness constant was 19 circ-1 in controls, 23 circ-1 in preoperative AS patients and increased slightly but not significantly to 33 circ-1 after valve replacement. Parallel to the postoperative decrease in systolic stiffness, muscle fiber diameter decreased significantly from 33 to 27 mu (P less than 0.05). The slight increase in diastolic stiffness was accompanied by a significant increase in interstitial fibrosis from 15% to 26% (P less than 0.05); LV fibrous content remained essentially unchanged after operation.(ABSTRACT TRUNCATED AT 250 WORDS)