Dynamic gadolinium diethylenetriamine pentaacetic acid (Gd-DTPA)-enhanced MRI was followed during growth and regression of MCF7 human breast tumors implanted in nude mice in the presence of estrogen and tamoxifen, respectively. Gradient-echo and spin-echo sequences were applied at a temporal resolution of 12 and 100 seconds, respectively, and a spatial resolution of 195 x 390 x 1000 microns. Maps of initial rates of contrast enhancement demonstrated stimulation of local growth of permeable microcapillaries at regions bordering necrotic areas, resulting from tamoxifen treatment. This localized angiogenic stimulation was confirmed by immunohistochemical staining of endothelial cells. After 1 week of tamoxifen treatment, the fraction of tumor pixels exhibiting rapid initial rate of contrast enhancement increased significantly from .28 +/- .05 to .46 +/- .06. In parallel, the fraction of tumor area showing contrast enhancement 3 minutes after Gd-DTPA injection also increased significantly, from .42 +/- .06 to .58 +/- .06. On the basis of these changes, it was possible to assess the response to tamoxifen therapy at an early stage.