We developed a new technique that monitors metabolic competency in female heterozygotes for ornithine transcarbamylase deficiency (OTCD). The method uses mass spectrometry to measure conversion of (15)NH4Cl to [15N]urea and [5-(15)N]glutamine following an oral load of (15)NH4Cl. We found that heterozygotes converted significantly less NH3 nitrogen to urea, with this difference being particularly obvious for symptomatic carriers, in whom the blood [15N]urea concentration (mM) was significantly less than control values at most time points. The blood concentration of [5-(15)N]-glutamine (microM) was significantly higher in both asymptomatic and symptomatic heterozygotes than it was in the control subjects. The administration of a test dose of sodium phenylbutyrate to the control group did not affect the rate of [15N]urea formation. We conclude: (a) This test effectively monitors in vivo N metabolism and might obviate the need for liver biopsy to measure enzyme activity in OTCD; (b) Asymptomatic OTCD carriers form urea at a normal rate, indicating that ureagenesis can be competent even though enzyme activity is below normal; (c) Although ostensibly asymptomatic OTCD carriers form urea at a normal rate, their nitrogen metabolism is still abnormal, as reflected in their increased production of [5-(15)N]glutamine; and (d) This new test may be important for monitoring the efficacy of novel treatments for OTCD, e.g., liver transplantation and gene therapy.