The role of beta adrenergic receptors in the control of venous O2 saturation heterogeneity was studied in the left ventricular free wall of 24 anesthetized open-chest dogs. Venous and arterial O2 saturation was obtained microspectrophotometrically and combined with radioactive microsphere blood flow determinations to calculate regional myocardial O2 consumption. The mean and variance of venous O2 saturation were determined under six experimental conditions designed to produce graded levels of beta adrenergic activity. Regional O2 consumption, extraction and flow were also determined. Propranolol, 2 mg/kg, significantly lowered the O2 saturation variance in both small subepicardial and subendocardial veins compared to control. Isoproterenol, 10 micrograms/kg/min, did not alter the variance of venous O2 saturation. Isoproterenol, 10 micrograms/kg/min, in the presence of 2 mg/kg of propranolol was able to prevent the reduction in variance of venous O2 saturation seen with propropranolol alone. However, 0.1 microgram/kg/min of isoproterenol did not prevent this fall in venous O2 saturation variance with propranolol. Practolol, 1 mg/kg, also caused a significant fall in venous O2 saturation variance in the subepicardium and subendocardium. From this it can be concluded that there is an inhomogeneous distribution in beta-1 adrenergic receptor population and/or activity in the left ventricle. Thus, there appear to be small regions in the heart with high O2 consumption and extraction related to beta-1 adrenergic activity and/or distribution.