The sustained release microspheres of theophylline were formulated using non-solvent addition technique. The in-vitro dissolution of the drug from the fabricated microspheres that having size ranges of 300-600, 600-800 and 800-1000 µm was tested. The release of theophylline was extended over 8 hrs and it was found that the drug release decreased nonsignificantly as the particle size increased (p≥ 0.05). Incorporating theophylline-containing microspheres into suppository formulation using polyethylene glycol base resulted in a slight increase in dissolution rate, but still in a sustained release pattern over 8 hrs. In-vivo study of the prepared suppositories on beagle dogs revealed that the peak of theophylline serum concentration Cmax (mean±S.D) was 11.1+0.3 µg/mL. It was also found that AUC(0–24hrs) value averaged 154.7±20.3 µg-h/ml. The median peak time (Tmax) was 3.0 hrs and MRT was 13 hrs indicating a sustained effect.