Transport limitations of critical nutrients are a major obstacle in the construction of engineered heart tissues (EHTs), and the importance of oxygen in this regard is well-documented throughout the literature. An indirect effect of cellular hypoxia is the shunt to the less-efficient glycolytic metabolism, which is accompanied by a reduction in extracellular pH. Image analysis of phenol red coloration in an experimental model of EHTs demonstrated pH gradients towards the center of the construct, which were dependent on experimental variables. Based on these observations, a four-species, 2-D diffusion-reaction mathematical model was developed to predict pH in a radial-diffusion model. The mathematical model predicted lethal values of pH (<6.5) in EHTs comprised of a nominal cell density of 10(6) cells/cm(3). pH predictions were moderately dependent on O(2) concentration, and strongly dependent on cell density, CO(2) concentration, and diffusion path length. It can be concluded from this study that hypoxia-induced acidosis is an important element in the mass transport problem, and future experiments measuring pH with more sensitive methods is expected to further elucidate the extent of this effect.