5'-Nucleotides of A and U with the phosphate activated with 1-methyladenine generate RNA oligomers containing 40-50 monomers in 1 day in reactions catalyzed by montmorillonite. The corresponding monomers of C give oligomers that are 20-25-mers in length after a 9-day reaction. It was not possible to determine the chain lengths of the oligomers of G since they did not give well-defined bands on gel electrophoresis. Co-oligomers of A and U as well as A, U, G, and C were also prepared. The oligo(A)s formed were separated by gel electrophoresis, and the bands of the 7-39-mers were isolated, the 3', 5'-phosphodiester bonds were cleaved by RNase T-2, and the terminal phosphate groups were cleaved with alkaline phosphatase. HPLC analysis revealed that the proportions of A(5')pp(5')A, A, A(2')pA, and A(2')pA(2)'pA formed were almost the same for the long and shorter oligomers. A similar structure analysis performed on the oligo(U)s established that the proportions of U-5'pp(5')U, U, U-2'pU, U-2'pU(2)'pU, U-2'pU(2')pU(2)'pU, and U(2')pU(2)'pU(2')pU(2)'pU did not vary with chain length. The structural analysis of the oligomers of A revealed that 74% of the phosphodiester bonds were 3', 5'- linked a value slightly greater than 67% observed when imidazole was the activating group. 61% of the bonds in the U oligomers were 3', 5'-linked, which is almost 3 times greater than the 20% measured when imidazole was the activating group. The potential significance of these data to the origin and early evolution of life is discussed.