A new cell line was derived from primary culture of rat choroid plexus (RCP) by immortalization with the TSOri minus adenovirus. The selected clone expressed vasopressin Via receptors at a density of 64,000 sites per cell, and a K-d of 7.2 nM. Addition of vasopressin to the RCP cells induced a transient calcium peak comparable to Via receptor signalling in different expression systems. This [Ca2+](i) increase was dose-dependent with an EC50 of 22 nM vasopressin. Similar [Ca2+](i) increase was elicited by addition of serotonin, angiotensin II, endothelin-1, and bradykinin, Heterologous desensitization of Via receptor was observed in RCP cells exposed to the phorbol ester PMA or following stimulation of other receptors coupled to the phosphoinositide pathway. Positive immunolabelling with Factor VIII, Flt1 and CD 34 antibodies suggests that this new RCP cell line originated from endothelial cells of rat choroid plexus.