cAMP stimulates cell proliferation and Cl^–-dependent fluid secretion, promoting the progressive enlargement of renal cysts in autosomal dominant polycystic kidney disease (ADPKD). Intracellular cAMP levels are determined by the balance of cAMP synthesis by adenylyl cyclases and degradation by phosphodiesterases (PDEs). Therefore, PDE isoform expression and activity strongly influence global and compartmentalized cAMP levels. We report here that PDE3 and PDE4 expression levels are lower in human ADPKD tissue and cells compared with those of normal human kidneys (NHKs), whereas PDE1 levels are not significantly different. Inhibition of PDE4 caused a greater increase in basal and vasopressin (AVP)-stimulated cAMP levels and Cl^– secretion by ADPKD cells than inhibition of PDE1, and inhibition of PDE4 induced cyst-like dilations in cultured mouse Pkd1^–/– embryonic kidneys. In contrast, inhibition of PDE1 caused greater stimulation of extracellular signal–regulated kinase (ERK) and proliferation of ADPKD cells than inhibition of PDE4, and inhibition of PDE1 enhanced AVP-induced ERK activation. Notably, inhibition of PDE1, the only family of Ca²⁺-regulated PDEs, also induced a mitogenic response to AVP in NHK cells, similar to the effect of restricting intracellular Ca²⁺. PDE1 coimmunoprecipitated with B-Raf and A-kinase anchoring protein 79, and AVP increased this interaction in ADPKD but not NHK cells. These data suggest that whereas PDE4 is the major PDE isoform involved in the regulation of global intracellular cAMP and Cl^– secretion, PDE1 specifically affects the cAMP signal to the B-Raf/MEK/ERK pathway and regulates AVP-induced proliferation of ADPKD cells.