Euglena grown to stationary phase in the dark without aeration accumulated lipids. When these high lipid cells are transferred to an inorganic medium and aerated, lipids were rapidly metabolized and the respiratory rate declined concomitant with the decline in cellular lipid content. Prolamellar bodies, propyrenoids and prothylakoids developed within the proplastid of dark aerated cells and the cells developed an increased capacity for chlorophyll synthesis manifested upon subsequent exposure to light. Lipid content did not decline in cells exposed to nitrogen and chlorophyll synthesis ability did not increase. The addition of an organic carbon source to cells at the start of aeration did not prevent lipid degradation. Organic carbon source addition and inhibitors of RNA and protein synthesis did however inhibit the development of an increased capacity for chlorophyll synthesis. These results suggest that oxygen triggers light independent proplastid development with the oxidative metabolism of lipids providing the carbon and energy for the synthesis of nucleic acids and proteins required for proplastid development in the dark.