An overview is presented of the results obtained with biodegradable sustained release devices (SRDs) containing a mixture of polymers and either isometamidium (ISMM) or ethidium. Under controlled laboratory conditions (monthly challenge with tsetse flies infected with Trypanosoma congolense) the protection period in SRD treated cattle could be extended by a factor 2.8 (for ethidium) up to 4.2 (for ISMM) as compared to animals treated intramuscularly with the same drugs. Using a competitive drug ELISA ISMM concentrations were detected up to 330 days after the implantation of the SRDs, whereas after i.m. injection the drug was no longer present three to four months post treatment. Two field trials carried out in Mali under heavy tsetse challenge showed that the cumulative infection rate was significantly lower in the ISMM-SRD implanted cattle than in those which received ISMM intramuscularly. Using ethidium SRD, however, contradictory results were obtained in field trials in Zambia and in Mali. The potential advantages and inconvenients of the use of SRDs are discussed and suggestions are made in order to further improve the currently available devices.