The aim of the present study is to perform stability study of ambroxol hydrochloride sustained release pellets stored in different storage conditions. The drug loaded beads were prepared by extrusion-spheronization technology then coated with ammonio methacrylate copolymer type A (Eudragit RL 30 D) and ammonio methacrylate copolymer type B (Eudragit RS 30 D) at a ratio of 2:3 (8% polymer by weight on dry basis) in fluid bed coater (Wurster column). Stability study of pellets was performed as capsule dosage form in aluminium-PVDC packaging mode at room temperature, 40 degrees C, 40 degrees C/75%RH & 30 degrees C/70%RH for three months. After one month the shape & size of the pellets was changed in all conditions. The color of the pellets remains unchanged up to the 2nd month in all conditions except at 40 degrees C/75%RH and in this case some pellets become brown. But after 3rd month, pellets become brownish in all conditions except at room temperature. At RT the color of pellets remains unchanged during the stability study. The mean drug content decreased gradually in all conditions. In acid media the initial drug release was 23% but after 1st month it was decreased to 13-15% in all conditions. In the buffer media (pH 6.8) the drug release was increased a little bit in all conditions except at 30 degrees C/70%RH with the passes of storage time. Stability studies at 30 degrees C/70%RH revealed consistent drug release (f(2)>50) throughout the stability period. The physical properties of pellets as well as the in vitro release profile of the drug was found to be a function of the different storage conditions as well as the physico-chemical nature of the polymers.