In nanopotentiometry a conductive atomic force microscope tip is used as a voltage probe in order to measure the distribution of the electrical potential on the cross section of an operating device. The information thus obtained is complementary to the carrier profiles and provides a method for calibration of device simulations. The experimental procedure is discussed in detail with emphasis on preparation techniques and contact force calibration. The present results are obtained on the cross section of a 0.25 mu m complementary mp,tal-oxide-semiconductor transistor cell which was designed such that even after sectioning, the device is still operational. I-V characteristics of the transistors before and after preparation indicate that the sectioned transistors show a higher leakage current, but are still functionally operational. Using nanopotentiometry, potential maps are measured under various bias conditions, showing the progressive creation of the channel. The present results demonstrate that nanopotentiometry provides insight into the actual working behavior of the transistors. The sensitivity and the spatial resolution are limited by the finite size of the conductive probe and by the quality of the surface preparation.