To study the effect of strong ac-fields (up to 100 kV/m at frequencies between 0.25 and 40 MHz), suspended or adherently growing animal cells were cultivated on micro-electrode structures. The use of miniaturised electrodes has the advantage that cells can be exposed to electric fields in physiological solutions of high conductivity. A disadvantage is that the actual field strength near a cell depends both on its location and on other parameters. The problem is that stray dielectric losses in and near the chamber change the electric field in an undefined, frequency dependent manner. This paper compares three different techniques for estimating the field: (i) impedance measurements, (ii) the measurement of dielectrophoretic forces and (ii) a new calorimetric method within a test chamber. Advantages, limitations and possible applications are discussed.