Superoxide dismutase (SOD) occurs in two intracellular forms in mammals, copper-zinc SOD (CuZnSOD), found in the cytoplasm, mitochondria and nucleus, and manganese superoxide dismutase (MnSOD), in mitochondria. Changes in MnSOD expression (as compared to normal cells) have been reported in several forms of cancer, and these changes have been associated with regulation of cell proliferation, cell death, and metastasis. We have found that progestins stimulate MnSOD in T47D human breast cancer cells in a time and physiological concentration-dependent manner, exhibiting specificity for progestins and inhibition by the antiprogestin RU486. Progestin stimulation occurs at the level of mRNA, protein, and enzyme activity. Cycloheximide inhibits stimulation at the mRNA level, suggesting that progestin induction of MnSOD mRNA depends on synthesis of protein. Experiments with the MEK inhibitor UO126 suggest involvement of the MAP kinase signal transduction pathway. Finally, MnSOD-directed siRNA lowers progestin-stimulated MnSOD and inhibits progestin stimulation of migration and invasion, suggesting that up-regulation of MnSOD may be involved in the mechanism of progestin stimulation of invasive properties. To our knowledge, this is the first characterization of progestin stimulation of MnSOD in human breast cancer cells.