Oxygen delivery and metabolism represent key factors for organ function in health and disease. We describe the optical key characteristics of a technique to comprehensively measure oxygen tension (PO2) in myocardium, using oxygen-dependent quenching of phosphorescence and delayed fluorescence of porphyrins, by means of Monte Carlo simulations and ex vivo experiments. Oxyphor G2 (microvascular PO2) was excited at 442 nm and 632 nm and protoporphyrin IX (mitochondrial PO2) at 510 nm. This resulted in catchment depths of 161 (86) mu m, 350 (307) mu m and 262 (255) mu m respectively, as estimated by Monte Carlo simulations and ex vivo experiments (brackets). The feasibility to detect changes in oxygenation within separate anatomical compartments is demonstrated in rat heart in vivo. [GRAPHICS] .