The detection of brain tumors using standard techniques of qualitative, relaxation-weighted magnetic resonance imaging (MRI) requires the application of contrast agents. We investigated whether or not it is possible to use diffusion-weighted MRI to localize tumors without contrast enhancement. Three different experimental rat brain tumors were studied: F98 glioma, RN6 Schwannoma and E376 neuroblastoma. We found a marked hypointensity in the region of the tumor and edema in heavily diffusion-weighted images, which corresponded well with the histological presentation. Quantitative maps of the apparent diffusion coefficient (ADC) allowed a better localization of the tumor than that obtained by regional presentation of T2 times, particularly under conditions in which peritumoral edema was absent. The ADC differences of the three tumor types were statistically not significant. Based upon regions-of-interest evaluations, tumor could be distinguished from peritumoral edema and normal brain tissue. However, a sharp demarcation between tumor and peritumoral edema was not possible, and this is attributed to a similar enlargement of interstitial space. It was concluded that diffusion-weighted MRI possesses a high potential for the detection of brain tumors but does not allow precise demarcation of the tumor border.