Isovolumic perfused rat hearts containing an intraventricular balloon were used to assess the effects of incremental balloon volumes on developed pressure, oxygen consumption, coronary flow, phosphorylation potential obtained by P-31 nuclear magnetic resonance, wall thickness obtained by two-dimensional echocardiography, and diastolic wall stress. Three phases in developed pressure were noted: (1) volumes from 0 to 150 μl resulted in a continuous increase in developed pressure; (2) with volumes from 150 to 250 μl, developed pressure remained constant whereas developed (systolic) and diastolic wall stress rose sharply; and (3) with volumes from 250 to 400 μl, developed pressure fell whereas developed (systolic) and diastolic wall stress continued to rise. The in [ (PCr) (Pi) ] was in synchrony with oxygen consumption at 0 and 50 μl balloon volumes, and then diverged at volumes greater than 100 μl. Oxygen consumption increased from 0 to 50 μl, was constant from 50 to 250 μl balloon volume, and then declined. The in [ (PCr) (Pi) ] fell precipitously at balloon volumes greater than 100 μl, most likely limited by oxygen consumption. Coronary flow did not change significantly until 250 μl or more of water was added to the balloon, and then it started to decline. Volumes greater than 100 μl result in overstretch of myofibers, as observed by the precipitous decline in in [ (PCr) (Pi) ], and the steep increase in diastolic wall stress. With excessive volume loading, the drop in phosphorylation potential, in [ (PCr) (Pi) ], appears to contribute to the decrease in developed pressure.