A polyacrylamide hydrogel undergoing a volume change was studied by using interferometric imaging. The method produces the radial distribution of the matrix density for the cylindrical hydrogel at different times during the volume transition caused by solvent change. When the water-swollen gel was immersed in a water-methanol mixture, contraction of the gel matrix occurred first in the edge and propagated to the center. The edge did not reach the equilibrium density immediately after the solvent change. The gel contracted rather evenly, although the edge maintained a higher density during the change. After reaching the uniform density, the center of the gel contracted further whereas the edge remained unchanged. A similar pattern was observed in the. reswelling in pure water. These findings indicate that the gel studied had a shear modulus not small compared with its bulk modulus.