Most of the studies on the putative membrane progestin receptor (mPR) alpha and beta subtypes that have been published in the 5 years since their discovery have supported the original hypothesis that they function as specific membrane receptors through which progestins induce rapid, nongenomic responses in target cells. Recent evidence that mPRalpha and mPRbeta have important roles in the regulation of oocyte meiotic maturation and sperm motility in both fish and mammals is reviewed. Although rapid, cell surface-initiated progestin actions on sperm to induce hyperactive motility have been demonstrated in several mammalian models, the identity of the membrane progestin receptor mediating this effect remains unclear. We demonstrate here that mPRalpha mRNA is expressed in human sperm by RT-PCR and that the mPRalpha protein is localized to the sperm membranes by Western blot analysis. Immunocytochemical staining of whole non-permeabilized human sperm confirmed the mPRalpha protein is expressed in the plasma membrane, and showed it is localized to the sperm midpiece, indicating a likely role of mPRalpha in progestin regulation of sperm motility. Moreover, the abundance of the mPRalpha protein on sperm plasma membranes from human donors that displayed low motility was significantly reduced compared to that on normal motile sperm. Finally, progestin treatment of sperm membranes caused activation of G-proteins. These results suggest that, similar to its proposed function in fishes, mPRalpha is an intermediary in progestin stimulation of sperm motility in humans by a mechanism involving G-protein activation.