An apparatus for coating pharmaceutical particles such as drug dosage forms or pellets is provided which includes a drum assembly mounted for rotation about a cylindrical axis of the drum and which includes an outer drum section and an inner perforated drum section mounted concentrically on an interior of the outer drum section such that an annular chamber is formed between the perforated inner drum section and the outer drum section. Two or more baffle plates are attached between an outer surface of the inner perforated drum and an inner surface of the outer drum which are configured to act as baffles to restrain or regulate the flow of air which define two or more compartments depending on the number of baffle plates. One end of the drum assembly includes an end plate with a central aperture for use in loading pellets into the drum and provide access to coating constituents that are pumped into the drum interior during the coating process. An air distribution plate is attached at the other end of the drum and has air flow passageways and is interfaced with an air introduction plate having air inlet and outlet passageways. As the drum rotates the air flow passageways in the distribution plate alternately come into flow communication with the air inlet and outlet passageways in the air introduction plate thereby providing a flow path of air into the perforate inner drum and out of the drum assembly.