Breast cancer patients frequently develop metastases. This process requires the degradation of extracellular matrix proteins which act as a barrier to tumour cell passage. These proteins can be degraded by proteases, mainly the matrix metalloproteinases (MMPs). MMP-2 and -9 which are frequently detected in breast cancer tissues. ProMMPs are released from cancer cells, and their activation is considered to be a crucial step in metastases development. In breast cancer, estrogen metabolism is altered favouring the accumulation of 2- and 4-hydroxyestradiol (2- and 4-OHE(2)). These estradiol metabolites can generate free radicals. Since reactive species are known activators of proMMPs, this study was designed to determine if the free radicals generated by 2- and 4-OHE(2) can activate proMMP-2 and -9. Activation of MMPs by hydroxyestradiol was determined by monitoring the cleavage of a fluorogenic peptide and by zymography analysis. Both estradiol metabolites activated the MMP-2 and -9. 4-OHE(2) was a more potent activator than 2-OHE(2), which reflects its higher capacity to generate free radicals. ProMMPs activation was mainly mediated through O(2)*-, although the free radical HO* also activated the proMMPs but to a lesser extent. ProMMPs activation was not observed with estrogens that cannot generate free radicals, i.e. estradiol, estrone, 2- and 4-methoxyestradiol, and 16alpha-hydroxyestrone. These results demonstrate that 2- and 4-OHE(2) at a concentration as low as 10(-8)M can activate the proMMP-2 and -9 and might play an important role in the invasion of breast cancer cells.