Phenacetin and lactose powders, as models of slightly and highly water-soluble drugs, respectively, were coated by means of the Wurster process.Phenacetin (80-250 mesh) was a fragile crystalline material and hence fluidization had to be moderate. As a result, the agglomeration of some particles or the adhesion of small particles to large particles could not be avoided. However, the particles coated with triacetin (plasticizer) -containing lacquer (60% as dry lacquer relative to the core material) released only 6.0% of phenacetin in JP XI disintegration medium No. 1. This result suggested that slight agglomeration did not prevent the encapsulation. When a polyethyleneglycol 6000 and polysorbate 80 mixture was used as a plasticizer in the coating of phenacetin, the membrane became more permeable than when triacetin was used.With lactose (100 mesh), which is a comparatively hard crystalline material, discretely encapsulated particles could easily be produced. The lactose particles coated with triacetin-and with polyethyleneglycol 6000-polysorbate 80-containing lacquer exhibited identical dissolution properties. Their dissolution profiles were characterized by a lag time, which increased with the amount of coating applied, followed by a rapid release. This lag time was the period needed for lactose crystals to be dissolved in rapidly taken up water. The inward flow of water should prevent lactose from diffusing out during the lag time. The high solubility and high permeation rate of lactose and the large specific surface area of particles produced by the Wurster process presumably account for the rapid release after the lag time. These dissolution profiles of coated lactose particles suggested that they should be useful as a model to study oral drug delivery systems with various lag times of dissolution.