The final step in urine concentration regulated by arginine vasopressin (AVP) occurs in the collecting duct. AVP signalling leads to the translocation of aquaporin-2-bearing vesicles to and fusion of the vesicles with the plasma membrane. Quantitative phase contrast imaging with digital holographic microscopy was applied to dynamically monitor changes in water permeability of primary cultured rat inner medullary collecting duct cells (IMCD) related to AQP2 regulation. In time-lapse measurements changes in cell swelling were investigated after application of a hypoosmolar shock (600 to 200 mOsmol/kg). Cell height was plotted against time followed by regression analysis. Untreated cells were compared to cells stimulated with AVP (0.5 mM, 30 min). Relative changes in cell height were described by sigmoidal curve fitting allowing determination of the time constant [tau]. After a hypoosmolar shock in control cells [tau] was 117 ± 5 s (n=9) and it decreased after AVP stimulation to 28 ± 1 s (n=27). The ratio of both time constants indicates a 4.2 fold increase in water permeability of the plasma membrane after AVP-treatment. The holographic microscopy is a well suited non-invasive and marker- free method to measure kinetics in cell volume changes and geometry with high temporal and spacial resolution to indicate alterations in water permeability.