Polyadenylate [poly(A)] sequences are associated with the 35 and 50S Newcastle disease virus (NDV)-specific RNAs as well as all six to seven of the 18-22S NDV-specific messenger RNAs extracted from infected chicken embryo cells. The poly(A) associated with the 18-22S RNA has an average size of 120 to 130 nucleotides. The 18-22S RNA synthesized in vitro by NDV's virion-bound polymerase contains six to seven species of the same size and relative proportions as its intracellular counterpart. This in vitro synthesized 18-22S RNA also contains covalently linked poly(A) sequences which, although variable in size, are usually larger and more heterogeneous than those from the infected cell. In vitro RNA synthesis is supported not only by magnesium (at an optimal concentration of mM) but by manganese (at an optimal concentration of 0.5 to 1.0 mM) as well. However, the major product made in the presence of manganese, although sedimenting at 18 to 22S, differs somewhat from the product made in the presence of magnesium.