Amino acid transport systems have been studied in isolated mouse oocytes throughout oogenesis. While these cells were lacking the A-transport system of Ehrlich cell (Christensen, 1975), they were found to be provided with the L- and ASC-systems of Ehrlich cell (Christensen, 1975). Since, in contrast to the A-system, the L- and ASC-systems mainly work by exchanging internal and external amino acids, we propose that their main function is to actively maintain the appropriate internal balance between different amino acids within the growing oocyte.