The site of feedback inhibition of the biosynthesis of pyrimidine nucleotides de novo was investigated in the isolated perfused rat liver. Hepatic uridine phosphate contents were specifically depleted by use of D-galactosamine. The effective activities of enzymes involved in the synthetic pathway were deduced from the rats of incorporation of labeled precursors into the acid-soluble uracil nucleotide pool and into some intermediates of the pathway. The labeling of hepatic urea was also monitored. When the uridine phosphate contents were less than 20% of controls, the incorporation of [14-C]-bicarbonate was stimulated about 20-fold. Label from [U-14C]oxaloacetate used as permeable precursor of intrace-lular aspartate was introduced into the uridylates to the same extent in normal and UTP-depleted livers. Similar results were obtained with labeled carbamoyl phosphate although the uptake of this compound by the liver was rather low. The lack of labeling of urea from exogenous carbamoyl phosphate does not indicate a free exchange of extra- and intramitochondrial carbamoyl phosphate. [ureido-14C]Ureidosuccinate produced in normal and D-galactosamine-treated livers almost identical labeling patterns of dihydroorotate, orotate and orotidine 5'-phosphate. The steady state concentrations of these intermediates were all below 15 nmol/g liver wet weight.