The rate of breakdown and reutilization of urea in man has been measured in five normal and two septic patients using 15N and 13C labeled ureas. The labeled molecules of the 15N urea dose were distinguished from the labeled molecules of the recycled urea by analyzing in a mass spectrometer the isotopic nitrogens produced when the recrystalized urine urea was treated with a hypobromite solution. In a normal subject with regular nitrogen intake, it was found that only 4/5 of the produced urea was excreted in urine and the rest was endogenously degraded. Seventy percent of the nitrogen and 63% of the carbon of the degraded urea were returned to the urea pool. On a nitrogen-free diet or after neomycin treatment with regular diet in the normal, the extent of urea splitting is considerably reduced. In the septic patients, breakdown, as well as recycling of urea was almost eliminated. It appears that the reate of endogenous catabolism of urea depends mainly on the activity of the gut flora which may be affected by dietary intake and clinical status of the subject. The method developed here could be applied for the quantitation of urea dynamics under different physiological and pathological conditions.