The present study is a detailed analysis of the determinants of the rate of fall of left ventricular pressure in the isolated canine heart. The determinants to be examined include physiological factors: heart rate, peak systolic pressure, systolic fiber shortening, and diastolic volume; pharmacologic agents: catecholamines and digitalis glycosides; and pathophysiological factors such as myocardial hypoxia and ischemia. Initial studies demonstrate that the fall in pressure between 95% of peak pressure and the pressure equal to end diastolic pressure can be fit to a function of the form P equals e At2 plus Bt plus C (r greater than .97) under isovolumic conditions. Beginning at the point of most rapid fall in pressure (peak negative dP/dt) pressure appears to fall in a near exponential fashion (r equals .97 or greater) for points taken at 4.0 msec. intervals. Indexes of ventricular relaxation derived in a similar manner from the rate of fall in pressure during the isovolumic diastole will be examined in a working animal preparation in relationship to the rate of diastolic filling during early diastole. Also the relationship of the rate in fall of pressure to the duration of the periods of diastole will be studied in the working canine left ventricular preparation and then in man.