The arrest of meiotic prophase in mouse oocytes within antral follicles requires the G protein G s and an orphan member of the G protein–coupled receptor family, GPR3. To determine whether GPR3 activates G s , the localization of Gα s in follicle-enclosed oocytes from Gpr3 +/+ and Gpr3 −/− mice was compared by using immunofluorescence and Gα s GFP. GPR3 decreased the ratio of Gα s in the oocyte plasma membrane versus the cytoplasm and also decreased the amount of Gα s in the oocyte. Both of these properties indicate that GPR3 activates G s . The follicle cells around the oocyte are also necessary to keep the oocyte in prophase, suggesting that they might activate GPR3. However, GPR3-dependent G s activity was similar in follicle-enclosed and follicle-free oocytes. Thus, the maintenance of prophase arrest depends on the constitutive activity of GPR3 in the oocyte, and the follicle cell signal acts by a means other than increasing GPR3 activity.