A systematic investigation concerning uraemic toxicology has been made using modern analytical methods. The interest is especially focused on substances which are present in uraemic subjects in high concentrations, the so called "uraemic toxins". The samples from biological fluids have been screened with special regards to the presence of peptides and low molecular proteins. These peptidic uraemic toxins are thought to be responsible for certain physiological disturbances when present in uraemic subjects.