The design of a microencapsulation procedure for the preparation of biodegradable BCNU-loaded microspheres used as intracerebral implants is the aim of this work. This approach will give sustained high local concentrations of the anti-tumour drug in the brain without the associated significant systemic toxicity. The microencapsulation technique used is a solvent-evaporation process based on the formation of an oil-in-water emulsion. The stability of BCNU in methylene chloride saturated with water and its high value of partition coefficient between methylene chloride and water justifies the selection of this organic solvent as the dispersed phase in the methodology. A spectrophotometric method for the quantification of BCNU in mixtures containing PLAGA. is developed which allows the evaluation of drug photodecomposition. The volume of methylene chloride and the concentration of PVA in the external aqueous phase are the two variables that induce the largest variations of the microsphere size. The two main process parameters leading to the highest microencapsulation yield are the polymer concentration in the organic phase and the volume of the external aqueous phase; whereas the pH of the external aqueous phase and the use of co-solvents in the organic phase lead only to a small improvement in microsphere payload.