Average blood flow (q) was determined by trapping of 15 micron radioactive microspheres in the vastus lateralis, the gastrocnemius-flexor digitorum superficialis and the triceps brachii of five 18 kg untrained mongrel dogs at rest and during graded treadmill running. Oxygen uptake (VO2) and cardiac output (Qco) were simultaneously determined. q leveled off in all investigated muscles at 60-100 ml X 100 g-1 X min-1 when VO2 was ca. 70% of peak VO2. Qco increased linearly with VO2 up to peak VO2. The regional blood flow (qR) distribution pattern within the muscle was found to be extremely scattered around q, both at rest and at heavy exercise. qR ranged from approximately 5 to approximately 55 ml X 100 g-1 X min-1 at rest and from approximately 10 to approximately 200 ml X 100 g-1 X min-1 at maximal exercise. No significant topographic pattern was observed in the qR distribution of the gastrocnemius muscle which was essentially similar to that previously found for the isolated-perfused muscle preparation. The results indicate that maximal limb muscle blood flow and/or its uneven distribution may be the primary limiting factor to peak VO2 in untrained running dogs.