In cancer research, tumor spheroids are a well established system to study tumor metabolism resembling the situation in vivo more closely cell monolayers. Spherical aggregates of malignant melanoma cells (MV3) and their invasion into rat brain aggregates have been investigated by quantitative NMR microscopy. Relaxation times (T1, T2) and diffusion parameter images were acquired with an in-plane resolution of 14 x 14 microns2. The authors were able to demonstrate that the morphology of the spheroids can be visualized on these NMR maps. The contrast was mainly manifested in relaxation maps, where average relaxation times T1 = 1.94 +/- 0.17 s and T2 = 42.8 +/- 6.3 ms were obtained for proliferating cells, and T1 = 2.49 +/- 0.31 s and T2 = 104.3 +/- 29.4 ms for the necrobiotic center. The mean diffusion coefficients were 0.59 +/- 0.12 micron2/ms and 0.85 +/- 0.14 micron2/ms, respectively. The authors could follow the dynamic process of tumor cell invasion in the investigated co-culture system. Knowledge about tumor cell migration and tumor cell invasion is essential for the understanding of cancer and its therapy. Quantitative NMR microscopy can study this dynamic process noninvasively and therefore may help to assess the influence of therapy on the micromilieu of these spheroids.