Aciclovir (acicloguanosine) has been demonstrated to be effective in the treatment of intraocular pathologies such as herpes simplex virus retinitis and acute retinal necrosis. Although intravitreal injections have been used with fewer side-effects than intravenous administration, the risk of complications increases with the frequency of intravitreous injections. For this reason, a biodegradable drug-delivery system, such as microspheres, able to promote prolonged release of the drug, offers a good alternative to multiple intraocular administrations. In this work, aciclovir-containing poly (D,L-lactide-co-glycolide) microspheres were prepared by the solvent evaporation method. Seven additives were incorporated in the microspheres to modulate the in vitro release rate of the drug: four non-fatty substances (polyethylene glycol 300, polyethylene glycol 1500, hidroxypropyl methylcellulose and gelatin) and three fatty substances (isopropyl myristate, vitamin E and Labrafil(R) M 1944 CS). Morphology of microspheres was evaluated by scanning electron microscopy. Granulometric analysis showed that particle size distribution was significantly influenced by the incorporation of additives. Loading efficiency decreased when fatty substances were added, whereas non-fatty additives promoted higher incorporation of the drug. Infrared and differential scanning calorimetry analyses indicated that microspheres prepared by the solvent evaporation process were not influenced by the type of additive used. In all cases, the initial burst resulted less than 5%. Additive-free microspheres showed a slow release within the first days, but when additives were incorporated, in general, the release rates of the drug were increased. Best release results were obtained for gelatin-containing microspheres. The release of aciclovir from these microspheres was adjusted to a zero-order kinetic from 1 to 49 days with a release constant of 1.13 mug/day/mg microspheres. A dose of 0.74 mg microspheres would be therapeutic for the herpes simplex and Epstein-Barr viruses (MIC 0.1 mug/ml) and 7.4 mg for varicella zoster virus (MIC 1 mug/ml) treatment in an animal model.