The influence of different values of arterial oxygen partial pressure (PO2) on local tissue PO2 was investigated. Tissue PO2 was measured on the surface of the beating hearts of rabbit and cat by the multiwire surface electrode as described by Kessler and Lübbers (21). In parallel experiments, local hydrogen clearance applying the H2-PH2 probe (33) was used to determine the mean blood flow per area (microflow) at the capillary level. Mean blood flow per area, v, was calculated from PH2 clearance curves obtained by local application of rectangular hydrogen pulses. The results are presented as histograms. Under steady state conditions (barbiturate narcosis), tissue PO2 ranged from 5 to 65 Torr (0.67 to 8.67 kPA) with a median of 31 Torr (4.13 kPA) in the cat heart. Mean flow per area covered values between 25 and 11 mum/s with a median of 55 mum/s. For rabbit heart muscle, the median was v = 37 mum/s and the range 28 to 46 mum/s. Hyperoxia broadened the range of tissue PO2 and shifted flow per area to lower values. Different degrees of hypoxia shifted the PO2 histogram to the left (median PO2 14 Torr and 4 Torr, respectively; [1.87 kPA and 0.53 kPA]) and the flow histogram to the right.