Circulating arginine vasopressin (AVP) levels are elevated in congestive heart failure (CHF). However, the second messenger changes of the AVP system in CHF have not been explored. The aim of the present study is to determine whether there are changes in cAMP production in the AVP system in CHF. Cardiomyopathic hamsters of strain UM-X7.1 were used. Normal Golden Syrian hamsters were used as controls. IMCD cells were isolated from both group of hamsters. cAMP accumulation experiments were performed with AVP and forskolin stimulation in vitro. There was more cAMP release after stimulation by both AVP and forskolin in the cardiomyopathic than normal hamster IMCD cells. This illustrates that hypersensitivity of IMCD cells to AVP exists in cardiomyopathic hamsters. This may be partly explained by the presence of V2 receptor adenylate cyclase hyperactivity. The hypersensitivity of IMCD cells to AVP may be one of the factors causing fluid retention in CHF.