Microdialysis has been developed as a technique to collect biological substances continuously through a semipermeable membrane under physiological conditions with a probe attached on various animal organs. The heart, however, presents several difficulties in the application of this technique, because the heart is beating in vivo in contrast to other organs. I developed a flexibly mounted microdialysis probe technique, which involves synchronized movement of the tip of the probe with the beating heart to reduce tissue injury. With this technique, itbecome feasible to make stable and long-term measurements of interstitial biological fl uid substances. By appling the flexibly mounted microdialysis probe technique to the beating rat heart, I was able to monitor free radical generation and measure the level of adenosine during AMP perfusion that gives an index of the activity of ecto-5’- nucleotidase in the tissue. In the future, this technique will have versatile applications for studies to elucidate actualmechanisms of pathogenesis in heart disorders.