Abstract—Measurements of the incorporation of [14C]NaHCO3into orotic acid, uridine nucleotides and RNA in tissue minces establish the occurrence of the complete orotate pathway for thede novobiosynthesis of pyrimidines in rat brain. Selective inhibition of the incorporation of various radiolabelled precursors into orotic acid by uridine demonstrates the operation of a feedback control mechanism in brain minces and indicates carbamoylphosphate synthetase to be the site of inhibition; purine nucleosides were similarly found to inhibit thede novobiosynthesis of pyrimidines.The activity of the orotate pathway, as assessed by the rate of incorporation of [14C]NaHCO3into orotic acid, was found to be very high in fetal brain and to decline rapidly with neurological development; the mature rat brain exhibits less than 1% of the activity of the fetal brain at 18 days of gestation. Comparative studies on the ability of minces of the brain and several extraneural tissues to utilize [14C]NaHCO3and [14C]aspartate as precursors of orotic acid lead us to speculate that variations in the ability of tissues to synthesize orotic acidde novoare determined by similar variations in their ability to synthesize carbamoylphosphat