Hollow microspheres (microballoons) floatable in JPXIII no. 1 solution were developed as a floating controlled drug delivery system. Microballoons (MB) were prepared by the emulsion solvent diffusion method utilizing enteric acrylic polymers dissolved in a mixture of dichloromethane and ethanol. To assess the usefulness of the intragastric floating properties of MB in sustained pharmacological action, nonfloating microspheres (NF) possessing riboflavin release profiles equivalent to those of MB were prepared. Riboflavin powder, riboflavin-containing MB, and riboflavin-containing NF were administrated orally to each of three healthy volunteers. Riboflavin pharmacokinetics was investigated via analysis of urinary excretion of riboflavin. As a result, although urinary excretion of riboflavin following administration of MB was not sustained in the fasted state, urinary excretion of MB was significantly sustained in comparison to riboflavin powder and NF in the fed condition. Additionally, the excretion half-life time (t(1/2)) of MB was prolonged significantly by feeding. Furthermore, MB provided significantly high total urinary excretion (%) of riboflavin compared to NF in the fasted and the fed conditions. In view of the finding regarding similar riboflavin release profiles of MB and NF in JP XIII no. 1 solution (pH 1.2) and no. 2 solution (pH 6.8), the intragastric floating properties of MB appeared to be beneficial with respect to sustained pharmacological action.