SummaryA convenient synthesis of leflunomide and its primary pharmacologically active metabolite A77 1726, each labeled with four 13 C atoms has been achieved. Starting from 13 C 4 -ethyl acetoacetate, each step of the synthesis proceeds in 60-82% yield. Analysis of the mass spectra of the labeled and unlabeled materials demonstrates that the compounds prepared herein will be suitable as analytical standards for bioavailability studies.