The effects of varying the concentration of Mg++ in the incubation medium on the contractile responses of rat isolated seminal vesicle to 5-hydroxytryptamine (5-HT) were investigated in the absence and presence of EDTA. Preparations incubated in Mg++-free and Mg++ excess media showed supersensitivity and subsensitivity to 5-HT respectively. In the presence of EDTA alterations in the sensitivity to 5-HT produced by varying the concentrations of Mg++ were comparatively less. Preparations obtained from reserpinized animals showed subsensitivity in normal and Mg++ excess media and supersensitivity in Mg++-free medium. In the presence of EDTA, reserpinized preparations showed slight supersensitivity in normal Mg++ medium, marked supersensitivity in Mg++-free medium and lesser subsensitivity in Mg++ excess medium. Probably EDTA by more effectively removing Mg++ from the membrane binding sites by chelation makes the membrane permeable to Ca++ leading to supersensitivity to 5-HT (observed only in the presence of EDTA). These results suggest that the failure of reserpine to induce supersensitivity to 5-HT in rat seminal vesicle may be due to an enhanced antagonism of Mg++ on Ca++ movements in this preparation due to the poor capacity of rat tissue to retain Ca++ (Krishnamurty and Grollman, 1976).