Dynamic geometry of the left atrium was studied in open-chest dogs instrumented with ultrasonic dimension gauges for the measurement of the transverse diameter together with left ventricular dimensions and left atrial and ventricular pressures. Three definite phasic changes of the left atrial diameter were observed during the cardiac cycle at the control state; shortening of left atrial diameter with atrial contraction, continuous chamber expansion during ventricular ejection, and either a reduction in the diameter or a plateau during mitral valve opening to onset of the next atrial contraction. Acute onset of mitral regurgitation initially induces a remarkable augmentation of atrial shortening with chamber dilation as a result of an optimal use of the Frank-Starling mechanism of the atrial muscle. When mitral regurgitation is progressively increased, the extent of the atrial shortening and expansion is diminished despite the geometrical advantage of a further increase in atrial diameter, indicating that this extreme dilation no longer provokes the Frank-Starling response and the atrial myocardium is made to operate on a descending limb of function. The amount of regurgitation is highly dependent on the geometry of the mitral orifice and a decrease in regurgitation with vasodilator therapy or with positive inotropic agents may be largely related to a decrease in the size of the left heart cavity, which brings closer together components of the mitral apparatus and increases its competence.