A novel pH-dependent gradient-release delivery system was developed by mixing three kinds of pH-dependent microspheres. Nitrendipine, a dihydropyridine calcium antagonist, was selected as the poorly water-soluble model drug. To obtain gradient-release of the active drug in the stomach, duodenum and lower segment of the small intestine, respectively, three kinds of pH-dependent polymers, i.e. Acrylic resins Eudragit E-100, Hydroxypropylmethylcellulose phthalate and Hydroxypropylmethylcellulose acetate succinate, were formulated to produce the microspheres, which dissolve at an acid condition, the pH of > or = 5.5 and > or = 6.5, respectively. The quasi-emulsion solvent diffusion method was employed in the manufacturing process for the microspheres. All three kinds of microspheres had a highly spherical shape and high incorporation efficiency (>91.0%). The particle sizes were mainly affected by the agitation speed and temperature of the manufacturing process. The results of X-ray diffraction suggested that nitrendipine in the microspheres was molecularly dispersed in an amorphous state. The drug dissolution behavior of the system under the simulated gastrointestinal pH conditions revealed obvious gradient-release characteristics. The dissolution profiles and content of the systems stored at a temperature of 40 degrees C and a relative humidity of 75% were unchanged during a 3-month period of accelerating storage conditions. The results of the bioavailability testing in six healthy dogs suggested that the pH-dependent gradient-release delivery system could improve efficiently the uptake of the poorly water-soluble drug and prolong the Tmax value in vivo.