Nifedipine has been incorporated into Eudragit RL microspheres by the emulsion solvent-evaporation process using an acetone/liquid paraffin system. The drug in the Eudragit RL microspheres was present in either molecular dispersion or amorphous state. The drug release profile from microspheres conformed to the Higuchi equation. The microspheres were stable under different storage conditions for six months with respect to drug content, dispersing state and release profile. The in vivo results in eight healthy volunteers showed that the relative bioavailability of the nifedipine sustained-release microspheres against commercial retard tablet as a control was 102.5%, and the microsphere formulation was bioequivalent to the retard tablet. Significant in vitro and in vivo correlation was also found for the microspheres.