The short proton relaxation times in the MR images of malignant melanomas make them different from most other tumors. We have previously shown that the T1 and T2 signals may be significantly influenced when the water distribution of the tumor is changed in vivo and in vitro. In the present work T1 and T2 were estimated and compared with the electron microscopy picture in subcutaneously implanted B16 melanomas in mice. Two hours after the mice were given an i.p. injection of 0.9% NaCl containing 10% glucose and 10% fructose (9 mice) both the T2 components were markedly and the T1 slightly prolonged. At the same time the electron microscopy picture displayed swelling of the melanocytes together with a marked decrease in number and size of their mitochondriae. There were no changes in the MR image or the melanocyte structure in control mice injected with 0.9% NaCl (9 mice) or 0.9% NaCl containing 10% fructose. It is concluded that the changed MR image may be coupled to the metabolism in melanoma.