The influence of non-active ingredients in the manufacture of pellets on in-vitro dissolution rate and on bioavailability of hydrochlorothiazide has been studied. Pellets were formulated using either microcrystalline cellulose or microcrystalline cellulose-carboxymethylcellulose sodium blends as matrix, and hydrochlorothiazide as the active ingredient. In-vitro drug release from the different pellet formulations was retarded in comparison to a conventional tablet formulation and was dependent on the nature of the non-active ingredient and, for the microcrystalline cellulose-carboxymethylcellulose sodium blend, of the dissolution medium. In-vivo bioavailability of both pellet formulations was low compared with that of the conventional tablet and the plasma concentration-time profiles did not suggest slow release.