Regional cerebral blood flow (rCBF) values were measured in nine normotensive subjects with known previous myocardial infarctions using 15 mu 11C-methylalbumin microspheres and positron emission tomography (PET). Microspheres were injected directly into the left ventricle of each subject during routine cardiac angiography and blood flow calibrated using the reference sample technique. rCBF values were compared with those obtained for a group of fifteen age-matched normal controls using the C15O2 steady-state inhalation technique. Using 1 cm radius circular regions of interest, the 11C-microspheres approach yielded mean blood flow values of 51 ml/100 ml/min and 48 ml/100 ml/min for regions of interest dominated by temporal and frontal cortical grey matter respectively. An rCBF value of 32 ml/100 ml/min was obtained for regions of interest dominated by frontal white matter. Mean rCBF values obtained for these regions using the C15O2 method were not significantly different (52 ml/100 ml/min, 44 ml/100 ml/min, and 28 ml/100 ml/min respectively), but the C15O2 approach gave a significantly lower rCBF value than the 11C-microspheresfor regions of interest dominated by occipital grey matter. Although the two groups of subjects studied were not strictly equivalent, the good agreement between blood flow values obtained using the 11C-microspheres and the C15O2 techniques is of interest, and suggests that the assumptions of the C15O2 steady-state approach do not lead to large errors in practice.