Reticulocytosis was induced in rats by injecting phenylhydrazine, a potent oxidizing agent. Red cell morphology was analyzed by scanning electron microscopy. The majority of red cells from rats given injections of phenylhydrazine were types 2 and 3 echinocytes. Stomatocytes were also observed, but pitted lobular reticulocytes were not detected. Echinocytes have not previously been observed in reticulocyte populations. In the reticulocytes, the relative levels of 16:1 and 18:1 were significantly greater than in erythrocytes. These differences in monoenoic acids may be due to the presence of endoplasmic reticulum, the site of desaturase activity in reticulocytes. Of all the fatty acids, the polyunsaturates are the most susceptible to attack during peroxidation. However, the polyunsaturated fatty acid composition of reticulocytes was similar both to that of erythrocytes and to reported values of young erythrocytes isolated by density. Therefore, it is unlikely that lipid peroxidation caused the formation of echinocytes.