Large porous microparticles of PLGA entrapping insulin were prepared by solvent evaporation method and evaluated in diabetes induced rat for its efficacy in maintaining blood sugar level from a single oral dose. Incorporation of Eudragit L30D (0.03% w/v) in the external aqueous phase resulted in formation of pH responsive enteric coated polymer particles which release most of the entrapped insulin in alkaline pH. At acidic pH, release of insulin from uncoated PLGA microparticles and Eudragit L30D coated PLGA microparticles was 31.6 +/- 21.8% and 17.5 +/- 1.29%, respectively, for initial 30 min. However, in 24 h, in vitro released insulin from uncoated PLGA and Eudragit coated particles was 96.29 +/- 1.01% and 88.30 +/- 1%, respectively. Released insulin from composite polymer particles were mostly in monomer form without aggregation and was stable for a month at 37 degrees C. Oral administration of insulin loaded PLGA (50 : 50) and Eudragit L30D coated PLGA (50 : 50) microparticles (equivalent to 25 IU insulin/kg of animal weight) in alloxan induced diabetic rats resulted in 37.3 +/- 11% and 62.7 +/- 3.8% reduction in blood glucose level, respectively, in 2 h. This effect continued up to 24 h in the case of Eudragit L30D coated PLGA microparticles. Results demonstrate that use of stabilizers during PLGA particle formulation, large porous particle for quick release of insulin and coating with Eudragit L30D resulted in a novel oral formulation for once a day delivery of insulin.