The antidiuretic hormone arginine vasopressin (AVP) mediates the calmodulin-dependent increase in urea permeability in the rat renal inner medullary collecting duct (IMCD; [1]). The urea transporter (UT) proteins UT-A1 and UT-A3 are located in the IMCD [2, 3]. It is therefore likely that one, or both, of these proteins is involved in the calmodulin-dependent increase in urea permeability in this portion of the renal tubule. In this study, the effect of the calmodulin inhibitor W-7 on transepithelial urea flux by monolayers of Madin-Darby canine kidney (MDCK) cells stably expressing rat UT-A1 was investigated. In the absence of AVP, transepithelial urea flux was 1.64 ± 0.34 (mean ± S.D.) nmoles/cm²/min (n = 4). Under these control conditions DMU-sensitive urea flux was 0.80 ± 0.22 nmoles/cm²/min (n = 4). In the presence of 100nM AVP transepithelial urea flux increased to 4.41 ± 0.66 nmoles/cm²/min (P<0.01; ANOVA; n = 4) and DMU-sensitive flux was 3.01 ± 0.25 nmoles/cm²/min (n = 4). In the presence of the calmodulin inhibitor W-7, AVP-stimulated DMU-sensitive urea flux was reduced significantly to 1.14 ± 1.00 nmoles/cm²/min (P<0.01; ANOVA; n = 4). In contrast, the presence of Calphostin-C (PKC inhibitor), CKII (Casein kinase II inhibitor) or H-89 (PKA inhibitor) had no effect on AVP-stimulated, DMU-sensitive urea flux (P>0.05, ANOVA; n = 4). These data suggest AVP regulation of the UT-A1 urea transporter is dependent on calmodulin, supporting a role for UT-A1 in the calmodulin-dependent regulation of urea permeability in the rat inner medullary collecting duct. However, further studies are required to confirm this and to elucidate the molecular mechanisms involved.