The instant invention is directed to a multiparticulate osmotic pump, for the controlled release of diltiazem L-malate to an environment of use, said pump comprising:(I) a carrier medium which does not maintain its integrity in the environment of use;(II) a multiple of tiny osmotic pump elements each comprising:(A) a core comprises diltiazem L-malate and an effective buffering amount of sodium bitartrate surrounded by(B) a rate controlling water insoluble wall, having a fluid permeability of 6.96.times.10.sup.-18 to 6.96.times.10.sup.-14 cm.sup.3 sec/g and a reflection coefficient of less than 0.5, prepared from:(i) a polymer permeable to water but impermeable to solute and(ii) 0.1 to 60% by weight, based on the total weight of (i) and (ii), of at least one pH insensitive pore forming additive dispersed throughout said wall,wherein the total amount of diltiazem L-malate in all of the osmotic pump elements is a therapeutically effective amount.