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Myocardial fibrosis and abnormal myocardial collagen content are common in many forms of pathological cardiac hypertrophy, including that mediated by pressure overload. Recently, in an experimental animal model of chronic aortic regurgitation (AR), we found a strong relation between myocardial fibrosis and congestive heart failure development. To determine if these fibrotic lesions are composed of collagen, as they are in pressure overload, and to determine if potential preventive therapies should be developed similarly in both diseases, we assessed left ventricular collagen content at three time points after AR induction. Moderate to severe AR was induced in 19 New Zealand White rabbits by inserting a catheter through the carotid artery to perforate the aortic valve leaflets. Animals were killed (1) when they showed echocardiographically discernible systolic dysfunction or (2) if normal cardiac function continued, either early (1 month) or late (>3 years) after operation. Fourteen age-matched, sham-operated controls and seven normal unoperated rabbits also were studied. Collagen concentrations were determined biochemically by hydroxyproline measurement. Fibrosis was measured histologically using Mason's trichrome stain and the fibrous collagen-specific stain, Picro-Sirius Red. Our results show an age-related increase in left ventricular collagen concentration with no specific increase among animals with evidence of fibrosis. We conclude that, unlike pressure overload, volume overload produces fibrotic lesions not composed predominantly of excess collagen and that the therapy needed to prevent fibrosis may be different in these conditions. Further study is needed to define the chemical characteristics of the fibrous lesions and the pathophysiological importance of this finding.