Gadolinium (Gd)-containing lecithin microcapsules were composed of a lactose core (75-90 mu m) coated with polyvinylpyrrolidone (PVP), a drug layer of Gd compound and PVP, and a membrane consisting of soybean lecithin, cholesterol, stearic acid and PVP. As Gd compounds, gadopentetate dimeglumin (Gd-DTPA-DM, MC-1), gadopentetic acid (Gd-DTPA, MC-2) and a stearylamide derivative of Gd-DTPA (Gd-DTPA-SAm, MC-3) were used. The microcapsules were prepared with a high coating efficiency and a narrow size distribution, 149-177 mu m. The release Gd-DTPA-DM and Gd-DTPA from MC-1 and MC-2, respectively, in a 0.9% saline solution was similar in spite of their difference in water solubility; it was delayed with a short lag time of about 10 min, followed by a rapid release and finally sustained; on the other hand, the release was remarkedly delayed and prolonged without the initial rapid release in the dextran solution. The release of water-insoluble Gd-DTPA-SAm was almost completely suppressed until 120 min in both aqueous solutions (MC-3). In the saline solution, all types of Gd-containing microcapsules were bursting devices at the initial stage, accompanying the delayed but high particle swelling, and thereafter diffusion-controlled drug-release devices. In the viscous dextran solution, they remained diffusion-controlled drug-release devices due to poor water uptake and poor swelling. These results suggested the possibility for using Gd-containing lecithin microcapsules in Gd neutron-capture therapy.