The installation of Plexiglas ultraviolet filters is described, designed to provide protection to paintings which are exhibited under diffuse daylight illumination at the National Gallery of Art, Washington, D.C. A photograph is shown of the placement of sheets of a special colorless ultravioletfiltering Plexiglas over the tempered diffusing-glass in the gallery ceilings. Brief mention is made of the investigations of photochemical damage being undertaken by the National Gallery of Art Research Project at Mellon Institute, Pitts burgh, Pa. One of the results of this research was the decision to provide the added protection to the paintings by installation of ultraviolet-filtering Plexiglas over the entire system of diffusing-glass ceilings in the galleries illuminated by natural daylight.