It has been demonstrated previously that urine flow in the recumbent near-term pregnant woman is appreciably less when she is lying on her back than when she is lying on her side and that without change of position the relative oliguria persists for several hours before it is overcome (1, 2).The present report extends the observations on the effect of position on urine flow to include studies of sodium and potassium excretion, effective renal plasma flow, and glomerular filtration rate in near-term normal pregnant women, in women with eclamptogenic toxemia of pregnancy, and in women with large abdominal neoplasms originating in the pelvis. Attempts have been made to determine the mechanism by which the various observed changes in renal function are produced. METHODSStudies were made 11 times in 10 near-term normally pregnant women and 13 times in 8 women with preeclampsia. In most of these studies the excretion of sodium and potassium, the effective renal plasma flow (RPF) and the glomerular filtration rate (GFR) were measured. In one normal and one preeclamptic pregnant woman the observations were repeated following the insertion of large-bore heavy-walled ureteral catheters into both ureters well above the level of the pelvic brim. Similar renal function studies were made in two women who had large abdominal neoplasms arising from the pelvis, one an ovarian serous cystadenocarcinoma and the other a uterus containing multiple fibromyomas. Both tumors were approximately the size of a uterus at 32 to 34 weeks' gestation. Seven patients who had either delivered 7 or 8 days previously or who had undergone total hysterectomy 9 or 10 days before were also observed. Three subjects of this group had been studied previously while pregnant.All medications were stopped the night before; 2 hours before starting the study, a breakfast of milk, cereal, and 'This study was assisted by a grant from the Cleveland Area Heart Society. toast was allowed. One-half hour later an indwelling catheter was inserted into the bladder and oral hydration consisting of 100 ml. of water every 20 minutes was initiated. At this time in the 17 studies where effective renal plasma flow and inulin clearances were to be measured sodium paraminohippurate and inulin were given intravenously in suitable priming doses, followed by a sustaining infusion of each administered intravenously at a constant rate of 4 to 5 ml. per minute. Urine was collected in periods averaging 20 minutes each. With each collection the bladder was washed with distilled water followed by air. In 14 of the studies only the clearances of endogenous creatinine were determined.Paraminohippurate was measured by the method of Bratton and Marshall (3), inulin by the method of Higashi and Peters (4), and creatinine by the method of Bonsnes and Taussky (5). Sodium and potassium were determined with the Beckman flame photometer.Usually the initial observations were made in the lateral recumbent position. One hour after starting oral hydration and one-half hour before starting the first ...