Theoretically, if blood is to have directional flow and different magnitudes of velocity, pressure should be distributed in the left ventricle (LV) as a function of both space and time. Thus, regional pressure differences (RPD) were examined in the LV of 20 open-chested dogs. High-fidelity transducers were implanted through stab wounds at three positions in the LV: the base (B), equator (EQ), and apex (APX). Simultaneous, high-fidelity LV pressures were compared in each region under rest conditions and during graded infusions of isoproterenol and propranolol. In the baseline condition, there were slight but significant differences between the APX and B in end-diastolic pressure (EDP), peak systolic pressure (PSP), peak dp/dt and Vmax. At rest, the largest RPD between the APX and B in PSP was 9 mm Hg (mean +/- 1 SE = 2.1 +/- 0.5), and the largest during diastole was 5.1 mm Hg (mean +/- 1 SE = 1.2 +/- 0.4); the largest difference in Vmax was 30.5% (mean +/- 1 SE = 10.8 +/- 2.4). During isoproterenol infusion the RPD in PSP were accentuated; the largest seen was 84 mm Hg between APX and B (mean +/- 1 SE = 15.7 +/- 4.5). The largest difference in Vmax between B and APX was 188% (mean +/- 1 SE = 48.5 +/- 9.4). Propranolol obliterated these RPD. These results indicate that there are significant RPD in the LV cavity; during systole the highest pressures decrease sequentially from the apex to the equator and to the base; during diastole these RPD are of lesser magnitude. These RPD affect all derived pressure indices, and these changes can be increaed or decreased by drug intervention. These results are important for two reasons: 1) the position of catheters in the LV cavity is important when pressure-dependent LV parameters are compared in different conditions; and 2) in any force balance analysis of the left ventricle it is important to note that force generation by the left ventricle is an active process that transmits regional pressure differences to the LV cavity.