We sought to determine if left ventricular (LV) function of the heart from the adult, chronically obese animal is impaired. Hearts from 50 wk-old genetically obese female Zucker rats (624 +/- 13 g) and their lean littermate controls (275 +/- 5 g) were isolated during ether anesthesia, supported metabolically by retrograde aortic perfusion (6 ml/min, 35 degrees C) with physiological solution containing suspended canine erythrocytes (hematocrit, 20%), and the ventricles were paced at 180 beats/min. A distensible, fluid-filled balloon was placed in the LV, and pressure-volume (PV) relationships were obtained. The obese and lean end-diastolic PV curves were not different, and therefore the obese and lean LV chamber compliances were similar. Comparison of the systolic PV relationships demonstrated that the obese rat's heart had a greater pressure-generating capability, which probably was a reflection of its increased LV mass (0.96 +/- 0.03 vs. 0.72 +/- 0.02 g). The calculated average meridional (or circumferential) peak systolic wall stress in the LV of the obese rat's heart, however, was significantly reduced compared with control. This diminished ability to develop systolic stress from the same end-diastolic volumes suggests that the hypertrophied LV of the middle-aged obese rat's heart is dilated or that its contractility is depressed, or both.