In eight normal mothers the effects of pyridoxal 5'-phosphate (PLP) addition on the plasma activities of aspartate aminotransferase isoenzymes (AST-s and AST-m) during labor and following delivery were investigated. The AST-s activities with and without reactivation by PLP appeared to increase immediately after delivery and they were even higher on the 4th day postpartum. On the other hand, there were significant elevations in both the AST-m activities immediately and at 2 h after delivery, but not on the 4th day postpartum. Of AST isoenzymes measured in the nonstress test, only the relative activation rate of AST-m by PLP added was significantly higher than the control value. The present study may come to the following conclusions: 1) The relative activation rate of plasma AST-m activity by PLP may be a reliable index of vitamin B6 nutritional status during pregnancy. 2) The increases in AST-m activity with and without PLP added during labor suggest a minimal damage of mitochondria in skeletal, cardiac and uterine muscle cells. 3) The AST-s isoenzyme determinations with and without PLP may be especially useful as sensitive indication of erythrocyte and/or liver damage after delivery.