Instrumental aspects of capillary zone electrophoresis in closed systems are reported. The combination of fixed-volume sample introduction, utilization of double-beam UV absorbance detection and the use of fixed membranes for the separation of the electrode reservoirs from the separation capillary proved to be a reliable instrumental configuration for capillary zone electrophoresis. Dispersion of eluted zones was studied as a function of capillary length. Repeatabilities of migration times and peak areas were determined. Several examples of separations of mixtures of organic acids and nucleotides are given. The separation of diluted and ultrafiltrated serum from patients with chronic renal failure proved to be promising.