The understanding of tumor-associated cerebral edema involves an elucidation of the mechanisms involved in the altered distribution of water in the vicinity of cells. Changes in cellular macromolecules such as intracellular proteins, extracellular matrix components, and cell-membrane proteins may alter the water interactions in and around cells. The technique of pulsed nuclear magnetic resonance (NMR) gives a measure of the relaxation properties of protons in water molecules in such systems. The T1 and T2 relaxation times are increased in cerebral tumors and peritumoral tissue compared with normal brain. The in vitro study of cerebral tumors requires a tumor model that possesses the properties of the actual tumor under study. The C6 astrocytoma cell line has many of the properties of glioblastoma multiforme. An NMR study of C6 astrocytoma cells grown in monolayer, as spheroids of varying sizes and when implanted into rat hosts, has been undertaken. Results show that T1 and T2 relaxation times are not a static feature of the tumor cells but may reflect changing microenvironments that result from the contribution of a number of interacting factors present in the growing tumor.