Sapwood of Japanese beech <i>Fagus crenata</i> Bl., was exposed for periods of up to one year to natural light (direct passing through normal glass), mercury lamp light (direct, ultraviolet and visible fractions separated by filters), and diffused indoor light of various brightness levels. Color changes were measured using the coordinates of the L,a,b color solid, and the results are presented in a series of graphs. Relationships of color changes to some of the chemical constituents of the wood are briefly discussed.