The construction of a simple apparatus for capillary zone electrophoresis is described, consisting of an optical system allowing direct absorbance measurement in the capillary in UV light, an evaluating electronic module and a high-potential source. An attempt was made to achieve maximum sensitivity with a simple construction. In the electronics, care was taken to obtain a quiet baseline and to optimize the signal-to-noise ratio. That part of the noise which is of the frequency band of the signal is filtered off. Both suction and electrophoretic sample introduction are possible. According to operator's choice, the apparatus can be run under constant voltage or current and is protected against overloading. The high-potential electrode chamber contains separate buffer and sample compartments and its construction offers an easy interchange between the running and sampling positions. The applicability of the system to the separation of amino acids as phenylthiocarbamyl derivatives, peptides and both artificial and naturally occurring protein mixtures is demonstrated.