Collagen of the normal and hypertrophied rat left ventricle was successively extracted with neutral salt and dilute acid solutions, and pepsin digestion. The yield of dilute-acid soluble collagen was only 0.3 to 0.6% of total collagen; the solubility with neutral salt was even lower. Limited pepsin digestion permitted extraction of 50 to 65% of total collagen. The distribution of the various types of collagen molecules was analysed in pepsin-solubilised collagen in the presence of 3.6 mol·litre⁻¹ urea with the aid of electrophoresis on polyacrylamide gels. In all samples of nonreduced and reduced left ventricular collagen of the rat, disc patterns of pepsin-soluble collagen revealed the occurrence of dimeric and trimeric components, as well as aggregates of higher molecular weight. Such observations suggest the presence of an extensive interchain and intermolecular cross-linking network. Electrophoretic analysis of nonreduced and reduced pepsin-solubilised collagen also revealed heterogeneity of left ventricular rat collagen due to its occurrence as a mixture of type I and type III collagen. The proportion of type I collagen molecule components was substantially higher than that of type III components in all investigated samples of rat left ventricular connective tissue. Postnatal growth, aging and myocardial hypertrophy may affect the ratio of type I to type III components.