WHEN a solution of 7-hydroxypteridine1 was spotted on a sheet of Whatman No. 1 filter paper, irradiated with ultra-violet light (principally 254 mµ) for 5 min., and developed (by ascending chromatography) in various solvents, two spots appeared. One of these was identical with the starting material (unirradiated material was used as a control on the same piece of paper), and the other proved to be 5 : 6-dihydro-7-hydroxypteridine1. This reaction did not take place in aqueous solution (although 2 : 6-di-hydroxypteridine2 undergoes photochemical change in water), but it did if allyl-thiourea was present as a hydrogen donor (unirradiated controls were not reduced). 2- and 4-hydroxypteridine were found also to be photo-altered on paper; but the products have not yet been identified.