Tracer studies employing uranium enriched in the isotopes U{sup 234}, U{sup 235} have been carried out in six human subjects; four males and two females. The uranium, 6 micrograms to 70 micrograms per kilogram of body weight was given intravenously in the hexavalent state as uranyl nitrate. Each individual of the series received a single injection of the metal except for one who was given two widely spaced doses. The first of these was when his condition was normal and the second after an acidosis had been produced by ingestion of ammonium chloride. Renal function tests including urinary catalase, protein, amino N to Creatinine N ratio and clearances of mannitol and p-aminohippurate were done before and after administration of uranium. Only at the 70 microgram per kilogram level in Subject 6 was there a slight rise in urinary catalase and protein suggesting that tolerance had been reached. The excretion of uranium was mainly in the urine, where from 70 to 85% of the administered dose appeared in the first twenty-four hours. Urine of the second twenty-four hours contained about 4% and the third twenty-four hour urine, 1.5% of the administered dose. Detectable amounts were excreted for at least two weeks.