Ureido [14C] succinic acid and orotic [2-14C] acid, which are metabolic intermediates for nucleic acid pyrimidines, were synthesized by an improved method of Nyc and Mitchell starting with aspartic acid and urea [14C], which is more easily available than potassium cyanate [14C]. The yield of orotic [2-14C] acid was 15.7% based on urea [14C].