This study had two objectives: (a) to determine the accumulation of collagen and its structural remodelling in the hypertrophied rat left ventricle after 4 and 8 weeks of abdominal aorta banding; and (b) to correlate these findings with the diastolic stress-strain relation of the intact myocardium. In comparison to age and sex matched controls, the collagen volume fraction of the hypertrophied myocardium after 4 and 8 weeks of aortic banding increased significantly from 3.5(SD1.0)% to 7.8(4.2)% and 6.2(2.0)% respectively. This accumulation of collagen, or fibrosis, occurred in the absence of myocyte necrosis. Scanning electron microscopy showed increased density and thickness of the collagen weave and tendons. At 4 weeks, light microscopy showed interstitial oedema and disrupted collagen fibrils. Left ventricular diastolic stress-strain relations of both pressure overload groups were significantly steeper than that of the control group. Thus the response of the interstitium to the hypertrophic process that accompanies abdominal aorta banding is a complex process that includes a structural remodelling of the fibrillar collagen matrix and the early appearance of interstitial oedema, each of which may contribute to a rise in the passive stiffness of the intact myocardium.