The instantaneous transverse diameter of the left atrium, left ventricular free wall segment length (SEG), and the long axis of the anterior papillary muscle (APM) length were measured throughout the cardiac cycle, using ultrasonic dimension gauges together with left atrial and left ventricular pressures in 12 open-chest dogs. During atrial contraction, left atrial diameter decreased from 19.7 to 18.7 mm, while left ventricular dimensions increased simultaneously. During ventricular ejection, percent shortening was 26% in SEG and 10% in APM, while atrial diameter increased continuously to 20.5 mm, with a concomitant rise in the v wave of left atrial pressure. After normal mitral valve opening, left atrial diameter decreased rapidly simultaneously with the y descent of atrial pressure. Graded mitral regurgitation was then produced by sectioning the chordae tendineae. With moderate mitral regurgitation, end-diastolic length of the SEG increased by 27%, while extent of shortening (AL) was augmented by 96%. End-diastolic length of the APM increased by 7%, and AL was augmented by 60%. Left atrial pressure was sharply elevated, with a distinct a wave followed by the more prominent v wave. End-diastolic diameter of the left atrium was enlarged to 22.9 mm with increased atrial shortening and expansion. As mitral regurgitation was increased to a severe degree by additional chordal rupture, end-diastolic length and AL continued to increase both in SEG and APM. Left atrial pressure was further elevated (a wave 25 mm Hg and v wave 47 mm Hg). Left atrial end-diastolic diameter further increased to 24.9 mm. However, the amplitude of left atrial shortening and expansion decreased remarkably. In severe mitral regurgitation, isoproterenol and nitroprusside decreased left atrial pressure and diameter, restoring more forceful atrial shortening.