The 75 congeners of the polyfluorinated dibenzodioxins (PFDDs) and about half of the 135 polyfluorinated dibenzofurans (PFDFs) have been synthesized by pyrolysis of fluorophenols and fluorobenzenes. The individual congeners were characterized by GC/MS. 2,3,7,8-TFDD was also characterized by 1H-, 13C- and 19F-NMR spectroscopy. The retention behaviour of PFDDs and PFDFs during gaschromatographic separation is entirely different from that of PCDDs/PCDFs or PBDDs/PBDFs. The PFDDs/PFDFs elute earlier than the PCDDs/PCDFs and the order of the elution is not governed by the degree of substitution, O8FDD eluting e.g. much earlier than the M1FDDs. A preliminary toxicological evaluation of 2,3,7,8-TFDD was carried out. The elimination of 2,3,7,8-TFDD from mice after a single i.p. injection is biphasic with a very rapid elimination half-life of 5 minutes and a slower phase of 165 minutes. This means a dramatically reduced half-life compared to 2,3,7,8-TCDD with 8.5 d. In liver the TFDD level reaches a maximum 30 minutes after injection and also declined in a biphasic manner. In rat hepatocytes a primary culture induction of CYP4501A1-catalyzed EROD activity could be demonstrated, indicating that 2,3,7,8-TFDD activates the dioxin receptor. In rat hepatocyte cultures similar EC50 values were found for 2,3,7,8-TCDD and 2,3,7,8-TFDD. So far no de novo synthesis of PFDD/PFDF could be detected under conditions were PCDDs/PCDFs are formed. Also, formation of PFDDs/PFDFs could not be detected during thermal treatment of fluorotrichloromethane or Teflon.