The purpose of this research was to study effects of butyl stearate (BS) on in vitro drug release of carboxyl terminal and ester end-capping PLGA-based tolterodine micro spheres.Introduction: Poly(D,L-lactide-co-glycolide) (PLGA) microspheres were widely investigated as sustained release systems and their drug release rates could be adjusted by adding tatty acid esters to oil phase during preparation [1].