The perinatal changes in renal hemodynamics have been studied in lambs. Eleven of the lambs were exteriorized, but maintained on placental circulation (fetal lambs). Eight of the lambs were delivered with cesarean section and studied immediately after clamping of the cord (newborn lambs). Nine lambs were delivered spontaneously and studied during the first 9 days of life. Renal blood flow (RBF) was determined by the microsphere method using a sample drawn from the iliac artery as the reference flow. The filtering capacity of the nephrons was evaluated after ferrocyanide injection and dissection of the nephrons. Total RBF did not seem to change much at birth but increased in relation to kidney weight during the first postnatal week. Clamping of the cord did, however, result in a change in intrarenal blood flow distribution, so that in the newborn lambs relatively more of the blood flow was perfusing the outer cortical region. During the first postnatal week there was a slight, but insignificant further relative increase in outer cortical blood flow. The relative increase in outer cortical blood flow at birth was accompanied by an increased frequency of filtering superficial nephrons from 22 to 77%. Practically all juxtamedullary nephrons were filtering before birth. Three days postnatally 98% of the superficial and 100% of the juxtamedullary nephrons were filtering.