The aim of this work was to develop sustained local release systems for radioiodinated iodo-2＇-deoxyuridine ((125)IUdR) from biodegradable polymeric microspheres to facilitate the controlled delivery of (125)IUdR to brain tumours. The selective uptake of IUdR into the cell nucleus results in cell disruption over the short range of the low energy Auger electrons. The biodegradable microspheres can be precisely implanted in the brain by stereotactic techniques and the IUdR within the microspheres is protected from degradation and thus a sustained source of radiolabelled IUdR is available in the vicinity of the residual tumour cells. Poly(lactic-co-glycolic acid), PLGA (85:15), microspheres containing cold IUdR and the Auger-electron emitter I-125, as (125)IUdR were prepared using the O/W, O/O and W/O/W emulsion-solvent evaporation methods. The W/O/W emulsion method was most effective in achieving good drug loading with the use of bovine plasma in the internal water phase. Also effective in improving the drug loading was the use of 20% acetone in the dichloromethane and the presence of Span 40 in the organic phase. Electrolytes (NaCl and IUdR) in the external acqueous phase also improved drug loading. After an initial rapid release from the microspheres, a sustained release was observed over 15 days for the ＇cold＇ IUdR. The sustained release portions of the release curves showed Higuchi (t(1/2)), diffusion controlled release kinetics. The radiolabelled IUdR microspheres showed a burst release effect of 30-40% followed by a sustained release over 35 days.