We mapped the distribution of dissolved oxygen and mammalian cells in a hollow-fiber bioreactor (HFBR) using F-19 NMR T-1, relaxation time imaging measurements on an infused perfluorocarbon probe molecule and diffusion-weighted H-1 NMR imaging of water. This study shows how cell density influences dissolved oxygen concentration in the reactor and demonstrates that NMR can play an important role in defining the biochemical engineering parameters required for optimization of HFBR design and operation.