The effect of a number of metabolic inhibitors on the calcification of Gloeotaenium loitlesbergarianum Hansgirg, a freshwater green alga, was studied. The inhibitors used were methylamine, trimethylamine, mercuric chloride, imidazole, fluoride, arsenate, atrazine, DCMU and dinitrophenol. The effects of these inhibitors show that transport, or stimulation of respiratory carbon dioxide evolution inhibits calcification. Calcification in Gloeotaenium is, at least partly, due to a local rise in pH as a result of photosynthetic carbon dioxide assimilation. There is also some evidence that, apart from its role in carbon dioxide assimilation, photosynthesis supplies the additional energy needed for calcification.