Pulsed field gradient NMR methods have determined the temperature-dependent diffusion of myoglobin (Mb) in perfused rat myocardium. Mb diffuses with an averaged translational diffusion coefficient (DMb) of 4.24-8.37x10(-7)cm2/s from 22 degrees C to 40 degrees C and shows no orientation preference over a root mean-square displacement of 2.5-3.5 microm. The DMb agrees with the value predicted by rotational diffusion measurements. Based on the DMb, the equipoise diffusion PO2, the PO2 in which Mb-facilitated and free O2 diffusion contribute equally to the O2 flux, varies from 2.72 to 0.15 in myocardium and from 7.27 to 4.24 mmHg in skeletal muscle. Given the basal PO2 of approximately 10 mmHg, the Mb contribution to O2 transport appears insignificant in myocardium. In skeletal muscle, Mb-facilitated diffusion begins to contribute significantly only when the PO2 approaches the P50. In marine mammals, the high Mb concentration confers a predominant role for Mb in intracellular O2 transport under all physiological conditions. The Q10 of the DMb ranges from 1.3 to 1.6. The Mb diffusion data indicate that the postulated gel network in the cell must have a minimum percolation cutoff size exceeding 17.5 A and does not impose tortuosity within the diffusion root mean-square displacement. Moreover, the similar Q10 for the DMb of solution versus cell Mb suggests that any temperature-dependent alteration of the postulated cell matrix does not significantly affect protein mobility.