We present a new tool and measurement protocol based on Atomic Force Microscopy for the AC and DC electrical characterization of biological samples at the nanoscale. The new developments allow performing a variety of electrical measurements (DC conductivity, A C small signal impedance) both in spectroscopic mode (i.e. in a single point) as well as in imaging mode (i.e. by providing a map of the electrical properties with nanometric spatial resolution) with minimum sample damage. Successful test measurements have been carried out on Purple Membrane. © 2007 IOP Publishing Ltd.