Due to technical challenges very little is known about absolute myocardial perfusion in humans in vivo during physical exercise. In the present study we applied positron emission tomography (PET) in order to 1) investigate the effects of dynamic bicycle exercise on myocardial perfusion, and 2) to clarify the possible effects of endurance training on myocardial perfusion during exercise. Myocardial perfusion was measured in endurance-trained and healthy untrained subjects at rest and during absolutely (150W) and relatively (70 % W_max) same bicycle exercise intensities. On average, the absolute myocardial perfusion was 3.4-fold higher during 150 W (p<0.001) and 4.9-fold higher during 70 % W_max (p<0.001) than at rest. At 150 W myocardial perfusion was 46 % lower in endurance-trained than in untrained subjects (1.67 ± 0.45 vs. 3.00 ± 0.75 mL · g^-1 · min^-1; p<0.05), whereas during 70 % W_max perfusion was not significantly different between groups (p=NS). When myocardial perfusion was normalized with rate-pressure-product (RPP) the results were similar. Thus, according to the present results, myocardial perfusion increases parallel with the increase in working intensity which is matching with increase in myocardial work rate. Endurance training seems to affect myocardial blood flow pattern during submaximal exercise and leads to more efficient myocardial pump function.