Effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) were determined at constant intervals during and after the normal pregnancies in 25 healthy women. Compared with non-pregnant values, ERPF increased by 80 percent during early pregnancy but fell significantly from this new level during the third trimester. GFR, however, remained at a level 50 per cent above the non-pregnant throughout pregnancy. Filtration fraction (GFR/ERPF) was significantly reduced during early pregnancy but rose to a value equivalent to the non-pregnant during the third trimester. Comparable data of previous workers are re-interpreted.