Dopamine is thought to exert a negative control on lactotrop cell proliferation and prolactin production. Indeed, mice lacking the D2 receptor develop pituitary tumors of lactotrop origin. Because lactotrops express two isoforms of D2R, D2L, and D2S, in a specific ratio, we decided to explore the physiological importance of their relative abundance in vivo. Thus, we generated transgenic animals overexpressing either D2L or D2S in lactotrops. Increased expression of D2S, but not of D2L, leads to mitogen-activated protein kinase (MAPK) induction, which results in pituitary hypoplasia. On the other hand, levels of phosphorylated MAPKs are drastically reduced in pituitary tumors generated by the absence of D2-dependent signaling. These results underline a critical role of D2-mediated MAPK activation in lactotrop proliferation. Furthermore, whereas D2S overexpression results to a drastic reduction of prolactin, D2L overexpression elevates it. Our findings underscore a different role of the two D2R isoforms in the pituitary gland physiology.