Left ventricular characteristics in 13 dogs with aortic regurgitation of 36 to 468 days duration were compared with those of 15 normal dogs. The test group exhibited significant chronic dilatation and hypertrophy and elevated left ventricular end-diastolic pressure, but contractility (tension—velocity relations) was normal in the group as a whole. In animals with the more severe and prolonged duration of lesions, however, contractility was depressed, although this was not reflected in greater elevation of left ventricular end-diastolic pressure.