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The sympathetic innervation of the hypertrophied myocardium was investigated by the histochemical fluorescence method for biogenic monoamines and electron microscopy. Cardiac hypertrophy was induced by renal hypertension, aortic stenosis and swimming exercise. An increase in the density of adrenergic structures was observed in relation to small vessels in the left cardiac wall in swimming exercised rats and, although to a much less extent, in the hypertensive rats. In relation to the larger myocardial vessels an increased density was observed in swimming exercised rats and in rats with long-term hypertension. In the electron microscope, nerve structures were seen adjacent to growing capillaries. The number of nerve fibres and filaments related to each vessel was not increased. No change was observed in the rats with aortic stenosis. The observations indicate that in cardiac hypertrophy secondary to swimming exercise the newly formed myocardial blood vessels aquire a normal adrenergic nerve supply.