Enhanced glycolysis represents a most striking feature of cancers and can therefore serve to indicate a malignant transformation. We have developed a non-invasive, quantitative method to characterize tumor glycolysis by monitoring ¹³C labeled glucose and lactate using magnetic resonance spectroscopy. This method was applied in MCF7 human breast cancer implanted in the mammary gland of female CD1-NU mice and was further employed to assess tumor response to hormonal manipulation with the antiestrogen-tamoxifen. Analysis of the kinetic data based on a unique physiological-metabolic model yielded the rate parameters of glycolysis, glucose perfusion and lactate clearance in the tumor, as well as glucose pharmacokinetics in the plasma. Treatment with tamoxifen induced a two-fold reduction in the rate of glycolysis and of lactate clearance but did not affect the other parameters. This in vivo metabolic monitoring can thus serve to evaluate the efficacy of new specific estrogen receptor modulators and may be further extended to improve diagnosis and prognosis of breast cancer.