Docking and fusion of vesicles to the plasma membrane is an essential process in living cells. From a geometrical point of view, this must lead to height alterations of the plasma membrane at the site of fusion. A well established cellular model for inducible vesicle trafficking consists of rat inner medulla collecting duct epithelial (IMCD) cells. Upon stimulation with the signaling peptide arginine-vasopressin (AVP), aquaporin-2 containing vesicles are inserted into the plasma membrane. Here, we used atomic force microscopy (AFM) to monitor nanometer height fluctuations in IMCD cells under physiological conditions (in PBS at 37°C). The roughness (Rms) was taken as an average measure for height changes. Control cells exhibited a Rms of 0.9 nm with no single frequency preferred. Stimulation with AVP leads to an increase in mean amplitude of 31% (to 1.2 nm Rms) while fixation of the cells with glutardialdehyde reduced the value to the instrument's noise (0.1 nm Rms). Occasionally, single height steps of up to 25 nm were recorded. They most likely represent single fusion events at the site of observation. We conclude, that membrane turnover causes height fluctuations of living cells, which can be monitored by AFM.