A diltiazem pellet formulation for oral administration comprises a core of diltiazem or a pharmaceutically acceptable salt thereof in association with an organic acid and a lubricant, and a membrane surrounding the core comprising a multiplicity of sequentially applied and dried layers, each layer containing a major proportion of a pharmaceutically acceptable film-forming, water insoluble, naturally occurring polymer and a minor proportion of a pharmaceutically acceptable film-forming, water soluble polymer. The number of layers in the membrane and the ratio of the water soluble to water insoluble polymer being effective to permit release of the diltiazem from the pellet at a rate allowing controlled absorption thereof over a twelve hour period following oral administration.