The success attending the use of azaserine as a specific inhibitor of one atage in the metabolic pathway leading to the synthesis of inosinic acid in pigeon liver prompted us to use this antibiotic in a similar attack on purine synthesis in Scenedesmus. However, investigation of the products produced during photosynthesis by suspensions of these algae in the presence of azaserine showed that a more widespread interference with metabolism had occurred. The purpose of this communication is to describe the nature of these effects and to attempt to assess their importance in a general picture of the metabolic effects of azaserine.