The relative amounts of glycine and serine, intermediates in the glycollate pathway in plants, that were 14C labelled in the light in six marine plants under different oxygen concentrations were measured to give an estimation of photorespiration. Glycine and serine were labelled most under the highest concentration of oxygen and least labelling occurred in the lowest oxygen concentration. These results support the theory that an increase in oxygen concentration increases the flow of carbon through the glycollate pathway, thus increasing photorespiration.